Understanding & Live Graphing of the Rear O2 Sensor: OBD4Everyone Ep.19
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- Опубликовано: 26 авг 2024
- On this episode of OBD 4 Everyone
we are going to explain and show everything you need to know about the rear o2 sensor AND show you how to use OBD Fusion LIVE graphing feature.
Almost every gasoline fueled car made in the last 25 years will have at least two o2 sensors located in the exhaust system.
One is located BEFORE the catalytic converter and it's main job is to provide feedback to the engine control unit on the exhaust gas concentration of oxygen.
Then the ECU will adjust the air/fuel mixture to maximize catalytic converter's efficiency and minimize emissions.
Now, the other o2 sensor is located after the catalytic converter, and this is the one we are going to be talking about.To make sure we all have the same understanding, there are multiple names for the rear O2 sensor
for example:
• downstream o2 sensor
• post cat o2 sensor
• and O2 sensor 2.
All of these names simply tells us it's located AFTER the catalytic converter.
If the air/fuel mixture is greater than 14.7 :1, or lean, the output voltage will be around 0.1 volts.
If the air/fuel mixture is less than 14.7 :1, or rich, the output voltage will be around 0.9 volts
and IF the air/fuel ratio is 14.7:1, the output voltage will be close to 0.5 volts.
This "narrow band" of sensitivity is why it's also known
as a "Narrow band O2 sensor"
Now, to determine how well the catalytic converter is working,
the ECU measures just about everything it can from the rear O2 sensor
like:
• how much time it takes to switch from rich to lean
• how much time it takes to switch from lean to rich
• the frequency of how often it switches
• and there's more, but I think you get the idea.I'll bet you are wondering, why does the air/fuel mixture change so much?
shouldn't it be more stable?
A few things come into play here...
When we accelerate, the mixture will be rich.
When we lift off the gas pedal to slow down, the mixture will be lean.
and when idling or driving at a constant speed
the ECU will change the air/fuel ratio back and forth between slightly rich and slightly lean.
while it analyzes how the rear o2 sensor responds to these changes.
A healthy catalytic converter has a high capacity to store oxygen,
and as a result of this, the rear o2 sensor should have a low switching frequency.
If it has a high switching frequency, the catalytic converter oxygen storage is low and it is not functioning as it should.
When the switching frequency is too low, the check engine light is turned on with a P0420 or P0430 which means Catalyst system
efficiency below threshold.Alright, let's wrap up this episode...
I hope you now have a better understanding of what the rear o2 sensors do and what their output should look like for different operating conditions.
And since most rear o2 sensors are the narrow band type,
everything shown in this video should apply to any gasoline fueled car
made in the last 25 years.
As always, thank you for watching and please subscribe.
Man Ive been a heavy line auto mechanic for 25 years plus years and that is the best information I've come across. very easy to understand and that was great as I needed a refresher
Hey Scott Pear,
Wow! Thank you for your kind words and I'm glad you found it helpful!
This video is insanely high quality. I really hope your content gets the visibility it deserves.
Hey Matt,
Thanks for your comment, that is very kind to say that!
As I have learned, my content is mostly viewed when people have a problem with their car AND not many people seem to work on their cars anymore... so my audience is a little limited in scope, which is OK :)
I've got a 2012 Toyota Corolla with about 115K miles on it. It has been well taken care of over the years by the previous owner, they changed the oil and such impeccably, had everything done on time, etc. As of the past couple of years, the downstream O2 sensor started testing only 1 or 2 out of the total of 5 readiness tests on OBD2, flipping between 1 and 2 out of 5 total tests for it and basically just halfway testing and untesting. I've checked voltage on the wiring, changed the O2 sensor 3 times in 2 years (Toyota-branded Denso made no difference vs aftermarket), had the battery tested, etc. When the ECU is reset completely, virtually all tests are complete the next day. It requires overnight testing for the Evap, but everything else tests very rapidly, including both readiness tests for the upstream engine-side O2 sensor. The catalyst also isn't testing, which I suspect is because of the downstream O2 sensor (before/on the catalyst) not testing first.
What could cause this? There is no check engine light, no error codes in the scanner or anything to indicate that anything is wrong. What am I missing? OBD2 device also shows the voltage of the downstream O2 sensor is registering and also it is reading the catalyst temperature properly. So I see that it is getting about 1V when the throttle is pressed, the upstream and downstream are talking to one another by adjusting fuel trim and such based on whether the throttle is being pressed, etc. Toyota recommended cleaning the throttle body, but that doesn't seem to be related to the downstream O2 sensor that we're having issues with. It's simply jumping back and forth between 2 of 5 readiness tests completed, then back to 3 or 4 out of 5 tested, then back to 2 or 3 tested and just says they haven't been completed yet. We've driven it hundreds of miles after reset. Same thing over again.
First of all, that's some good trouble shooting and testing! Good work!
A few things come to mind:
• Using an OBD scanner and a good app (like OBDFusion) view/graph the rear or sensor voltage. This will allow you to check for the typical swing of 0.1 to 0.9 volts.
• In most cases, the catalyst temperature is a calculated value based on engine load and airflow, so that may not tell you anything
• Find the drive cycle for your car. This is what the engine computer requires to get the monitors to run. All conditions need to be met for the catalyst drive cycle to run. In some cases weird things like radio off, more than 1/2 tank of gas etc...
You could also do a search to see of the is a Technical Service Bulletin (TSB) that may address this issue. In some cased a software up may be all it takes.
Good luck!
@@OBD4Everyone Thanks for the reply.
Yep, voltage goes between that range and increases when pressing on the gas pedal. Also the downstream O2 sensor heater test is among the tested things.
Tried the drive cycle thing, too. Even Toyota was clueless as to what to tell us other than, "That will be a very expensive problem to diagnose." They want to put it on a $158 diagnostic (their fancy word for the OBD2 scanner similar to what I'm using now and "drive it" to see what it is doing. Naturally, that won't fix it. I suspect that they would then start guessing at parts like the catalyst, suggest to replace the O2 sensor for a fourth time in less than 2 years, etc.
This is what I see when it tests the most tests, before going back to a couple of the green ones untesting again: i.postimg.cc/dQWrrP2R/screengrab.jpg
i.postimg.cc/q45ZHWPC/screenshot2.jpg
Someone told me to "tap on the catalytic converter and listen for rattling because it may have broken apart due to age". It's 10 years old. I had an old Ford Escort that we sold to my uncle after about 20 years (owned it from new) and he didn't have any trouble with emissions. Do they really go bad that quickly on Toyotas?
I don't have much experience with Toyota's.
The only other suggestions I have are to check for any pending codes and clear them.
Then do the drive cycle mentioned here:
justsmogs.com/2016/05/1996-to-2002-toyota-obd-ii-catalyst-monitor-readiness-drive-patterns/
axleaddict.com/auto-repair/Toyota-Drive-Cycle-Readiness-Monitors-for-Emissions
From what I have read, it looks like it can be difficult to get the catalyst monitor to run.
Good luck!
Great video! Really learned a lot.
My O2 S2 Sensor switches between 0,53V and 0,67V at idle with about 1.7s peak to peak frequency.
Not sure, but might be a bad catalytic convertor then.
I do have very rare rpm drops when approaching a light slowly to 500rpm and sometimes backfires at high rpm. :(
i have purchased the extended obf fusion for my rav4 2019 adventure, i sat the graph as per ur instructions and when in action the only grph that was working is the accelerator and the speed, the o2 sensors are not showing any graph though the engine temperature is above F180, the interface am using is ELM327 wifi, the reason am running this diagnosis is that when i drive for two hours or more i start hearing explosions in the exhaust, it might be also important to mention that the fuel we use most of the time is not good and stations mix it with water.
I would run a diagnostic report. This will show you all the supported sensors/PIDs and their name for your vehicle. Then select those exact descriptions in the live data section. Then you should see the data directly from the engine computer.
Truly great info. My question is: I have 2003 Land Rover Freelander 2.5L V 6 engine. It has also two cat. converters, two Front and two Rear O2 sensors as explain in the show. Last year my local shop cut out completly one of my cat. converters and weld in just a straight pipe instead and then cut out the end of the second cat and pierced trough a hole with a screwdriver (because they couldn't access its front) to cut completly out. They also installed some kind of adapters?? to correct check engine light issue. However, right now my check engine light comes on all the time after a few minutes of driving. Also, they told me that I have to remap my ECU to eliminate Rear O2 sensors so the check engine light won't come back again. I know this will be very risky procedure as ECU remapping can go wrong during. and my car won't derivable. What I should do to correct this situation? Are they are any solutions to correct this issue? Please let me know if you can. Thank you very much. S.F.
Hey Sylvester Furtak,
The proper and most likely the only legal fix is to remove the straight pipe and put in a new catalytic converter. That will keep your check engine light off and the engine computer happy. Yes, they are expensive!
If you don't have an emission test or visual inspection to pass other options are:
• Live with the check engine light
• Have the ECU programmed to ignore the incorrect signal from the post cat (sensor 2) O2 sensor.
If you choose to have the ECU re-programmed you might want to find someone who has done it before by searching for Freelander forums or FaceBook groups.
In Canada and the US, it is illegal to remove the catalytic converter with fines from $300 - $600.
Really good video! Thanks for explaining this so well.
Glad it was helpful!
Great narration!
Thanks! The screen was recorded while I drove and I added the voice over later. It was not a quick process, but I was very happy with how it turned out :)
Hi, my Genesis coupe has a Cat delete and an 02 housing installed. Ofc, because it doesn’t detect different readings with the sensors, throws P0420 code.
I tried doing the spark plug defouler method to avoid the rear 02 being directly in the flow of the exhaust, but it just swaps that code for others.
I want to see if I should clog or open up more the fouler to get the right voltage reading, if it’s reading on idle like 0.3, should I open more or clog the defouler to get it closer to 0.5-7 range?
Hey amazing video with great detail however im at a loss car was throwing code p0100 maf sensor replaced it engine ran so much smoother no more rough idle but now i get code p0170 fueltrim bank 1 astra h 1.8 2006
LTFT(B1) has a constant 11.7 o2s1 (B1) fluctuate between 0 and negative 25 o2s2 (B1) says negative 100 and doesn’t move any idea where to start i was thinking maybe o2 sensor is bad but im no car expert would be great full for any advice thanks in advance
As you can guess, it is difficult to troubleshoot over RUclips comments.
I don't have any first hand knowledge on GM products and I don't know if the O2 sensors are wide band or narrow band.
You might want to join a car forum or FaceBook group for the Astra and ask them.
Sorry, but I don't think I can be any more help.
2:48 "when the switching frequency becomes too low the computer throws the p0420 code." This should be when the switching frequency becomes too HIGH the computer throws the p0420 code, or it could be when the capacity to store oxygen becomes too low the computer throes the p0420 code
You are correct!!!
I can't believe I said that... I'll see what I can do to fix it.
Thanks for pointing that out!
@@OBD4Everyone Actually both statements are true for most modern cars. Back in the day you could just use a spacer to mute the response and fool the CPU. On new cars that doesn't work anymore, the CPU will throw a code if there is to much activity or to little. They have gotten wise to those simple tricks so now it looks for the response to be within a certain window. It is a lot harder to fake a healthy cat these days.
I have an Actron CP9690 code tester. It checks graphing. It also has emission readiness test. It has other testing abilities. EBAY has it.
I just check amazon.ca for the price of that unit.... It is not cheap at $289. I prefer the app based OBD scanners, as they allow easy data logging without a PC.
A Launch X431V V.40 tests just about everything on a car! It's $860.00! The Actron CP9690 works on OBD 1 and OBD II. It's for basic testing. It has code-connect to diagnose codes. It has emissions readiness test to check systems to pass an emissions test. It has graphing to check oxygen sensors and other systems. Select record and then touch the system twice to see graphing.
I've got a 2004 Volvo v40 2.0t and just got the p0420 code. There isn't any loss of power but I have noticed a smell of unburnt gas or something similar when the car is idling at a stop. I'm pretty sure it's the cat that's gone bad but the fact that there isn't any loss of power concerns me.
Hey Strikemi,
Don't confuse "catalyst air flow" and "catalyst efficiency." You can have a cat that flows well, but is not at the required level of efficiency.
Since you have a P0420 and you can smell the exhaust, this points to the catalyst.
I got error code P0420 and when analyzing the o2 sensors during a drive, the O2B1S1 was fluctuating as it should but the O2B1S2 would hold at around .75-.8v and then randomly drop to below .2v for a second or two at a time. Compared to the good side, B2S2 which always held pretty constant at around .75-.8v the whole time. Do you think this has to do with the sensor or the catalytic converter?
First off all, that is some good troubleshooting!
That is a tough question to answer...
I would think the rear O2 sensor is bad.
Does the engine make the same power under full throttle? If it does, the catalytic converter is not plugged and that would make me think the rear o2 is faulty.
Spray your throttle with brake cleaner and check if the o2b1s2 is changing voltage..if it is change voltage then yiu have priblem with your catalylic
Have an 03 Honda Element. Got a P0420, so I set up the O2 sensor readouts on Torque Pro. O2S1 reads fine, but O2S2 is flatlined at 0v no matter what the throttle position. Put in a new O2 sensor and the same. Never any change in the plot, just stays at zero. Would a bad cat cause flatlining, or is this more likely an electrical problem?
Hey NobleLock,
Since the voltage stays at zero, I would start by look at the wiring and check for any damage, then check for continuity from the O2 sensor to the ECU.
Also, check for any exhaust leaks. A crack can allow additional air into the exhaust stream and can result in a lean (low voltage) signal. BUT there would be some voltage level changes.
If the engine performance of the car is normal (and not lower) the catalytic converter is likely good.
Good luck!
Hi. If only my B1S2 sensor is showing small fluctuations (0.65 to 0.70), while all the other 3 sensor show normal fluctuations, does that likely mean the sensor is bad? I keep getting the P0420 code that comes back after I drive 1 full cycle every 3 days. My conclusion would be that the sensor failed.
Car is not old at all. I recently installed high flow cats with O2 spacers. Maybe the sensor got contaminated, got loose, or burned out somehow.
Hey ShinyBlackRims,
A P0420 can be set by the rear o2 voltage change not being enough, or the rear o2 voltage switching frequency switching too fast. The rear o2 switching frequency should be less than the front o2. (assuming your vehicle doesn't have a front air/fuel sensor)
The issue might be the rear o2 sensor... To test you could swap the rear O2 sensors (left to right, right to left) and see if the codes changes sides.
*This assumes you have 2 banks, which I think you do as you did mention have 4 sensors.
Newer vehicles are getting much smarter (the ECU/PCM programming) in detecting catalyst efficiency. In some cases, the O2 extenders don't "trick" them anymore.
Hi great video I have same problem you this is the downstream o2 sensor the is bad when I let the pedal off the show rich I press the pedal and the drop o to 10% the problem is when I go like 50 miles and let the pedal is when I have the problem thanks for your time
Hey Jose Alvarez,
I am having trouble understanding what the issue is.
Are you saying the rear O2 sensor reads rich (almost 1 volt) when you take your foot off the gas pedal?
Great video, full of useful information! If you dont mind me asking some questions I would like to go for it.
1. Is it possible to make sure that the CC is in the place (I mean not stolen or been removed by previous owner) by seeing the voltage on O2 Downstream sensor? I mean would the sensor still give voltage information on OBD2 if the CC is not there?
2. I have Peugeot 407 2.2 petrol. I just bought o2 sensor for downstream because I noticed weird movement (like very slow response) on o2 downstream sensor while checking with OBD2 so Im going to my mechanic to change it this friday but before that could you tell me that P0410 code is related to CC or O2 sensors somehow? My car is keep throwing p0410 code and I already replaced secondary air pump but still get the code. And I have awful gas milage in the city (long ride its normal), is this somehow also related to o2 sensor?
Thanks a lot in advance!
Hey Hurgeneral,
Thanks for your kind words!
I am not familiar with Peugeot's but will do my best :)
Answers:
(1) Yes. If the catalytic converter is functioning properly, the rear O2 sensors should be switching between slowly (2-5+ seconds) between lean (about 0.1 volts) and rich (about 0.9 volts). The front O2 sensors should be switching much quicker (about every 1-2 seconds)
(2) As mentioned above, a slow switching rear O2 sensor is a sign of a healthy catalytic converter.
As you know, the P0410 is a secondary air issue. There is an air pump that runs for 1-2 minutes on a cold start to add extra air into the exhaust to speed up the catalytic converter warm up to reduce startup emissions.
The engine computer will turn on the secondary air pump and then watch the fuel trims to ensure they go a little lean (due to the extra air being injected into the exhaust)
If you haven't already, check the relay for the secondary air pump, check for any small cracks in the secondary air lines and check for any exhaust cracks or leaks.
Good luck!
@@OBD4Everyone Thank you very much sir! Im a bit relieved about CC because its very common to remove it around here! Everything will be more clear on friday of course but I just couldnt wait :)
Anytime! I hope all goes as you hope :)
I have a problem with me 2013 honda crosstour v6. The car has 186k miles and every 2 days map sensor like come up p1129. I replace map sensor, plugs, timing belt check for ground, replace ecm but the light keep coming. When the like come the exhust smell like egg and no power at all at full throttle the rpm slowly go to 4k before changing. I took my upstream 02 out and i start the car for 5 minute immediatly inside the cat i can see red cherry color and i dont have code for cat or 02 sensor. I check 02 reading and the guy who help say my cat is bad he say the cat hold power back am lost. Before i order cat i need your help or input thanks
If your catalytic converter is glowing red, that indicates that a very rich mixture or even raw fuel in getting into the exhaust. This is very bad for the converter.
Also, it is possible that part of the catalytic converter is melted inside and blocking the exhaust.
My best guess, is that the converter is partly plugged (or melted), restricting the flow of exhaust and reducing engine power. This may also explain the MAP fault, as the increased exhaust pressure can elevate the intake manifold pressure.
If that was my car, I would replace the converter and verify that the air/fuel mixture is correct by looking at the sensors wave forms as shown in the video.
Good Luck!
Other sites/technicians tell that if the rear 02 sensor waveform is the same as the front 02 sensor waveform, then this is a catalyst failure for sure. In my case, I found a p0420 error, which I erase it. I monitored front and rear 02 sensors voltages and found that in the very start of the car, the rear waveform is flat. After some reving, I saw two identical waveforms, with pretty high frequency. However, I drove in a city with ups and downs regarding the acceleration. Is this normal behavior? OBD told me that the catalyst is not emission ready and the last temperature recorded was 300oC. Thank you in advance!
Hey Dimitris Peponis,
I agree, if the front and rear o2 sensors have the same/similar switching frequency, the catalytic converter function is low.
I find it odd, that your rear o2 sensor is flat upon start up, unless a cold engine is being started as it takes some time for the o2 sensor to get up to operating temperature.
The rear o2 sensor should switch slower than the front o2 sensor, if the catalytic converter is functioning well. If the rear sensor is switching (with proper voltage levels) too quick, that will set a P0420 code.
I think the best way to check switching frequency of an O2 sensor is to find a flat and long road where you can drive at a steady speed of 80KPH / 50 MPH for 3-4 minutes. The will keep the engine load about the same and then you can see what is going on. A change in engine load, will change the switching frequency.
To determine if how well the catalytic convert if functioning, maybe you could find a shop that has a 5 gas analyzer:
www.underhoodservice.com/tech-tip-using-a-five-gas-analyzer-to-diagnose-catalytic-converters-2/
This would confirm if the catalytic convert is the issue for much less money than buying a new converter and finding out it is something else. :)
@@OBD4Everyone Thank you for the response.
I noticed that the rear sensor voltage sometimes oscillates with frequency equal to 1/5 of the front one, some others is exactly the same. Flat line of rear sensor is only on cold start.
@@OBD4Everyone Thank you for the answer. Today the p0420 became permanent. However, I used the log function of the OBD fusion and the results in the CSV file do not agree with what I see in the monitor. The app monitor shows that the rear sensor oscillates with smaller amplitude and frequency than the front one. What's happening with the log procedure of OBD fusion?
Hey Dimitris Peponis,
That is a little odd... I wonder if the catalytic converter is cracked/loose and moving around. The rear o2 sensor should have a mostly constant switching frequency when at an constant load (flat road at highway speed)
Hey Dimitris Peponis,
I suspect the datalog data update rate is faster than what is displayed. Maybe check with OBDFusion support?
Thank you very much. I fully understand that replacing both cats in my car will be the best solution, however the cost of doing so will be much higher then the current value of my car. If I remember correctly our local shop quoted me for about 16 hours labor (times $130 per hours) and cost of two catalytic converters about $3000 each. So the total cost will be at least $ 8000 CAN. My car is still in an excellent condition. It is very clean and drives very well. I don't want to put it in the junk yard or part it out at this time. Also, we are not required locally to have cat converters to be installed on the car as our laws are not preventing us from driving a passenger car without cat. converters being removed.
My second question will be: How the temperature of the missing cat. converter will affect output voltage and its characteristics from the oxygen sensors? I measured my temperatures of both sides driver side (Bank 1) and a passenger side (bank 2) of my car and they are substantially different - about 50 C difference. My scanner shows only error on Bank 2 O2 sensor post-catalytic converter were my old catalytic converter was drilled out/pierced through with a screwdriver but doesn't show error on the driver side (Bank 1) where a straight pipe is welded in.Thank you fot you help S.F.
Hey Sylvester Furtak,
I completely understand your situation.
Your rear O2 sensor has a built in heater so it can get to the 300°C operating temperature quickly.
With this in mind, I can't see the exhaust temperature having any impact on the operation of the O2 sensor.
Have you looked into "O2 extensions" for the rear O2 sensors? They are inexpensive and might trick the ECU that the cats are functioning ok. Do a search on Amazon to see what they look like.
In a nutshell, the tubes extend the rear O2 sensors out of the exhaust stream, which slows down their switching from lean to rich and rich to lean, which is what the ECU looks for to verify a proper functioning cat.
Good luck!
OBD4Everyone they even have some of those 02 extensions that contain a catalyst within the e extension.
As i know if the rear O2S switching from rich to lean that’s means the catalytic converter is bad , it should read study voltage about 0.5 volts. Am I wrong?
There are two things to factor in.
(1) The catalytic converter stores and releases a little oxygen
(2) The air fuel ratio is varied from slightly rich to slightly lean for proper catalytic converter operation.
With the above in mind, the rear O2 sensor will switch from rich to lean / lean to rich, BUT it should be much slower than the front/pre-cat o2 sensor, due to the oxygen storage of the cat.
Anyways, that is my understanding :)
@@OBD4Everyone Very accurate explanation
Thank you so much
Thank you for your video, I have GMC terrain 2011 6 cylinder, I have code P0430 , I clean the sensor and I have same problem
Sensor 1 bank 2 give low voltage like 0.01 or 0.1
Sensor 2 bank 2 give 0.7
Most of garages tell I have problem with catalytic converter
Other garage he confirmed I have sensor problem, pls help me to take my decision..
Thanks
I don't have any first hand working knowledge on any GM product, so I don't know if I can help you.
Generally, "Cleaning" an O2 sensor is never required and may cause problems. Depending on what was used to clean it, it may of contaminated the sensor and altered its function/sensitivity.
If the sensor 2 voltage is not changing, you might want to graph the sensor 1 and sensor 2 voltages to compare.
BUT you need to know if the sensor 1 is an O2 sensor or a air/fuel ratio sensor, as the voltage outputs will be different.
Since I have never worked on GM products, that's all I can offer.
Hopefully someone else can offer a suggestion.
Hello sir, great video. I am using the same app and same technique. However, my vehicle is a Toyota Camry 2005 2.4L 4 cylinder engine. It has two oxygen sensor, upstream and downstream. My o2 sensor 1 (upstream) shows no voltage reading at all while idle or driving. However, o2 sensor 2 (downstream) moves around between 0.70 - 0.88 V while driving. I have a check engine light on with P0420, bank 1. Is it normal to have sensor 1 (upstream) at 0 voltage all the time? I am assuming it is dead and that is what causing the p0420 engine code. Please, advice. Thanks,
why not you buye a new upstream O2 sensor , it is very important to have one as it is the front line defense against high fuel consumption
@@AutoTechEngineeringDHAKarachi is there another way of checking upstream sensor? I have 4 cylinder vehicle and gas milages (mpg) are normal.
@@AutoTechEngineeringDHAKarachi i just checked the voltage of the front sensor manually and it shows no voltage. Thanks for the advice.
Before you troubleshoot or replace the front "O2 sensor" you need to know what type it is:
• O2 sensor that outputs a voltage of 0-1
• Air/fuel sensor that outputs a voltage of 0-5 volts depending on the air/fuel ratio. Nissan air/fuel senors output about 2 volts at a hot idle.
I have no idea what type of sensor your Toyota is using, but once you know, then you can accurately troubleshoot the problem.
The last thing I want for you it to replace a part that you didn't need.
Very knowledgeable information. Is normal to jump into open loop when descelarate and recover close loop just after step on gas pedal and recovering acceleration? O2S2 both banks stay on 0.8 mV and Fuel trims from that sensors stay on 99%. Any clue is everything normal?
Yes, that is normal. Open loop will happen under full throttle and when you lift off the throttle.
You don't have anything to worry about :)
Hi, and congrats for your video. One question. I have a Mercedes clc 200, i have the primary cat removed which is in the exhaust manifold, and changed the second cat with a high flow 200 cel cat. He installed also an o2 sensor fooler with a minicat in the downstream o2 sensor in order not to have codes. Will i burn more fuel due to downstream o2 sensor fooler or have any other problem? Thanks
Each manufacture defines their own strategy for emissions and emissions verification.
Some vehicles do use the rear O2 sensor for fuelling calculations and some don't.
As I see it, the only way to know, is to try it and see.
@@OBD4Everyone
⁷
Hey buddy, thanks for the video, appreciate it! I have been getting P0296 and P0420 for bank 1 (right side) on my 3.6 Grand Cherokee. I've checked rear and post cat sensors and figured out my CAT it dead (200k km), so I got Magnaflow California Legal universal CAT which everyone praises in forums to solve P0420. Indeed it helped, although P0296 did not disappear at all. I`ve checked voltage of post cat sensor Bank 1 plays between about 0.1-0.2v when driving steady, although bank 2 driver side (original cat) post cat sensor plays between 0.6-0.7v.
What do you think is the root cause?
If post cat bank 1 only gets to 0.2v, I would check for an exhaust leak/cracked weld. If possible, swap the post cat bank 1 and bank 2 sensors and see if the lean/0.2v moves to the next bank.
If it does, you might have a lazy/slow/dead o2 sensor.
Something you said @2:50 seemed to differ from what you graph said. I was confused. When the switching frequency becomes too low the catalyst efficiency is below threshold. But, low switching frequency means the catalyst's ability to store oxygen is normal.
Did you mean to say if the frequency is too high the efficiency code gets set?
Hey Tom,
As I understand it, there is an acceptable range of switching frequency.
Too high indicates poor O2 storage/low catalyst function and too low may indicate a failing rear O2 sensor or exhaust leak.
Remember the code description is the symptom which may or may not be the cause.
Often people read the code "catalyst efficiency below threshold" and assume the catalyst is bad. It may be bad, or a exhaust leak, or a failing rear o2 sensor.
Thank you for the video. It was very informative. Currently, I'm facing a problem with my truck throwing p0420 code which mean catalytic below efficiency. When i reviewed the data from both o2 sensors based on your video, my downstream o2 sensor constantly shows voltage reading of .8v while cruising at same speed. It goes down to .2v when i lift foot off gas. Should not it be close to .5 at cruising speed? Do I really need a new catalytic converter?
As I understand it, the PCM/ECU looks for voltage levels swings (0.1to 0.9 volts) as well as the frequency (how often) the voltage swings between high and low to determine the efficiency of a catalytic converter.
If the o2 sensor is good and there are no exhaust leaks/cracks to let in air/oxygen, the catalytic converter may be bad.
One option to verify catalytic converter function is to find a shop with a 5 gas, exhaust gas analyzer. See: www.walkerexhaust.com/support/tech-tips/five-gas-diagnostic-chart.html
Hi, thanks for your explaining video.
In my case, I don't understand the downstream sensor behavior. When starting cold it is absolutely flat, then starts to move gradually in voltage range and frequency.
About 5 minutes later, at 2000rpm it already varies from 0.3v to 0.6v with about half the frequency of the sensor 1.
10 minutes after start it goes from around 0.4v to 0.7v with the same frequency.
After a trip, at 3000 rpm, I registered values from 0 to 0.8v.
The sensor 1 has the normal behavior, ranges from 0v to 0.9v, usual frequency...
I guess this points out that the cat is not doing it's job but if it's the case, then why the cat just seems bad when it warms up?
It's a short journey car that makes just around 3000km / year.
Thanks in advance.
(As you can imagine, it is very hard to diagnose issues through a comment, but I will do my best)
If you don't have a check engine light, then the o2 sensor operation is OK.
However, I suspect you do have a P0420/P0430 code. If the switching frequency of the rear O2 sensor is similar to the front O2 (assuming the front O2 sensor is NOT a wide band / Air/Fuel sensor) that would indicate the catalytic function is not working well.
It can be difficult to determine a bad O2 sensor from a bad cat. If the vehicle has high mileage it is often best to replace the O2 sensor first and re-evaluate.
@@OBD4Everyone Thanks for your reply
I once had a P420 in the summer but never again. I guess the o2 sensor behaviour I described was because the o2 sensor was warming up. After warming up, the downstream sensor graph is very similar to the upstream one. Meanwhile I found too that the STFT and LTFT are negative. Even after a reset, the LTFT stays around -16. I tried too to check the cat temperature at the entry and exit but was somewhat difficult because of the shield. Do you think that running rich (whatever the problem is) can cause a false inefficient cat report, or the cat should deal with it anyway, if it were fine? And if the exit cat temperature is higher than the entry, should I assume the cat is ok?
Thanks
I'm kind of confused at how you're representing the results of the graph from 4:24 going forward. You (and others) say that the readings for the rear O2 sensor on a healthy converter should be flat and have a high change frequency but yet your graph on a supposedly healthy converter shows that the rear O2 sensor also go up and down and when you rev up the engine, it's identical to the 1st O2 sensor. Just how flat should the rear O2 sensor be? Is the ECM smart enough to allow for the 2 sensors to be identical when revving up the engine and not give you a P0420 code?
Reason I ask is that when I read my code, the rear O2 sensor reading sometimes mimics the 1st O2 sensor almost perfectly and I think "oh here we go, the cat is bad" and then other times when I'm idling or maintaining my speed it's relatively flat with a spike here or there. In my case, this is the 2nd aftermarket Cat I put in and each one lasted about 2 years (original lasted like 280K miles) so this time around I tried some cat cleaner in my fuel and to my surprise, I noticed it helping the graph and then my check engine light went away by itself but I'm not sure just how well things are working.
From what I have seen, the lower cost aftermarket cats don't work as well or as long as the OEM version. That seems to be your experience as well.
The frequency of voltage change of the rear O2 compared to the front (pre-cat) O2 sensor depends on the type of the front o2 sensor.
Today, most front O2 sensors are actually air/fuel sensors and output a voltage between 0 to 5 volts, so they don't switch like the old narrow band O2 sensors.
To troubleshoot this your self, you need to know what type of front O2 sensor does the vehicle have, what the voltage output is for a given air/fuel ratio and compare that to the rear or sensor voltage output.
@@OBD4Everyone Thanks for your response. This car is definitely not new! It's a 23 year old Toyota that just keeps on going!
The graph in the video doesn’t show the 1st O2 sensor. You can only see 2x sensor 2. Green = Bank 1 (behind the first catalytic converter) and red = bank 2 (behind the second catalytic converter).
Good job
Thanks
Good video!!!
Hey Autoolsee store,
Thanks again. It was interesting collecting, organizing and analyzing the data.
Great video really helps me understand what's going on in the exhaust system, but I'm in a bit of a weird spot where I have an incomplete catalyst monitor, but no check engine. I have replaced the 2nd o2 sensor, and driven around 200 miles. Still no completion for catalyst monitor, any tips on how I can look at post and pre cat o2 sensor graphs to get a sense of what's going on?
Hey Inner G,
The requirements to get monitors to run are different for different vehicles.
You need to find out the conditions that are required for the monitor to run and made sure they are followed. Then you need to make sure you follow the catalyst drive cycle for your vehicle.
It can also be helpful to look at the mode $06 data in OBDFusion (or OBDLInk) diagnostic report fort failed items.
Good luck!
Quick question - I've got a '98 Jeep Cherokee, only two o2 sensors with a p0420 engine code -
bank1s1 fluctuates constantly between 0.1 and 0.9
bank 1 sensor 2 is reads more smoothly, ie, not constant fluctuation and goes up with acceleration, and down upon deceleration
Is it most likely my catalytic converter is the issue as I am thinking, or perhaps my bank 1 sensor 1 that fluctuates constantly is at fault?
From what you describe, the sensor 1 switching seems normal. It should be fairly quick and between 0.1 - 0.9 volts.
You didn't mention the voltage levels of the sensor 2... it should also be about 0.1 - 0.9 volts, AND have a slower switching frequency. (but not too slow or too fast!!)
If you have a P0420 I think the least expensive first option is to replace the rear O2 sensor and re-evaluate.
4:30 If the oxygen out = oxygen in doesn't that mean the cat is not working properly?
More curious from my perspective, my. Odb2 only allows bank 2 current to be measured while bank 1 is voltage
A properly working catalytic convert will store oxygen for a short period of time.
If the O2 in = O2 out, the oxygen storage capacity is very low and typically a P0420/P0430 code will be set.
Unfortunately, some OBD PID descriptions are not accurate. I suspect that is the reason why bank 1 is voltage and bank 2 is current.
Helpful video. I didi use a BlueDriver to scan all my o2's voltages and ratios. I also have a V6 camry as well, but Toyota or not is not my question.
The graphs for both upstream sensors' voltages are very consistent with each other, however, the graphs for both downstream sensors' are different.
Bank 2's downstream sensor's graph seems to change steadily, kinda like hilly terrain.
Bank 1's downstream sensor's graph seems to rapidly fluctuate from 0 to 0.9 volts, kinda like spikes, or upside-down icicles.
Which downstream sensor is bad? I have replaced the catalytic converter on bank 2 due to a P0430 code, and that code went away.
Generally, the switching frequency (0 to 1v or 1 to 0v) of the rear o2 sensor should be slow at a steady engine speed and load with a healthy catalytic converter. (that means it has the ability to store oxygen)
As you have seen with your new cat on bank 2, you see a slow change from rich to lean / lean to rich. This indicates that the O2 storage capacity of the catalytic converter is good... as it should be, since it is new.
As a guess, if the rear O2 sensor is good, I would investigate the bank 1 cat. As a "over the internet guess", the wave form should be more like the new cat on bank 2.
@@OBD4Everyone Actually, with another Toyota I have, that has no issues whatsoever, I decided to scan its front and rear o2's.
Sure enough, the waveform from my other Toyota resembles the new cat's, although I'm hoping it's just the sensor going out, since that particular sensor is out of stock at most places.
That was a good test! Good luck on the fix :)
Do the output voltages of downstream o2 sensors affect operation of the engine? 2002 F150 with lean codes on bank 1 and 2 both. No o2 codes but OS1S2 is stuck higher than OS2S2 at idle.
Hard to say as each manufacture has their own strategy. I can tell you for a fact that my 2011 G37 does use the rear O2 sensor to check for a lean/rich bias of the upstream air/fuel sensor.
As you can guess, it is difficult to troubleshoot from the internet, but I would have a look at your fuel trims and see if they are more than 10% (adding fuel)... one possible option is a weak fuel pump.
I’m interested in installing an O2 on an older disse vehicle to look at AFR - is there a tool I can buy/rent to hook up directly to the O2 sensor to output that data? I’m not looking to have a permanent mounted gauge, I just want the data for tuning. Plus, most gauges don’t work for the higher AFR ratios of diesels
I don't know anything about diesel engine control systems or tuning.
If you want to read the AFR, you will need a wide-band air/fuel sensor and controller/display. I suspect the older diesel will only have a narrow band O2 sensor which isn't useful for tuning.
This article may help you:
www.hpacademy.com/technical-articles/tuning-a-diesel-youll-need-these/
I have no codes but a slight misfire and a bogg down when warm when I hit the gas bank one and two upstream are up and down bank one down stream is between .730-.750 but bank two downstream is between .025-.130
That is very odd... one back is a little rich and one is very lean.
If the engine performance is down during a full throttle acceleration test, there may be a problem with the catalytic converters. It would be odd for both to fail at the same time, however.
When the banks have extreme and opposite fueling, that can be caused by the rear O2 sensors being switched by accident and end up in the wrong bank. (bank1 sensor in bank 2)
Might be an exhaust leak. The ECU will wrongly think it's lean because air is reaching the sensor so it will over fuel and run too rich causing bogging etc
Thank you for the great information, would you please put the link for the obd2 that you been using please?
Good option: $100 OBDLInk MX+, cheap option $30 VeePeak BLE.
Hi .. thank you for this rich information ... i have chrysler pacifica 2020 .. i am in Iraq i had replaced a commerical catalyst converter cause it had been stole and the code p0159 raised ... the oxygen sensor 2 bank 2 read 0.7 and doesn't fluctuate... what should i do ? please
I don't have any experience working on Chrysler vehicles, but from what I could find out, the P0159 means the bank 2 rear o2 sensors is taking too long to switch from lean to rich or rich to lean. AND that is exactly what you have seen with the rear o2 at 0.7 volts. (good troubleshooting!!)
I would check for any possible exhaust leaks (cracks, broken exhaust flange gasket, etc) You could switch the rear o2 sensors (left > right, right > left) and see if the problems changes to the other side. If it does, the rear O2 sensor might of been damaged when the catalytic converter was stolen.
That's all I got for you... Good luck!
reading of 0.05 doesn’t really fluctuate
Bank 1s2 is 0.585
Bank 1s1 is 0
Bank 2s1 is 0.047
Getting fault code p2198
Any help would be great, I’ve only just recently replaced every o2 sensor. The problem existed before replacing these
Hey T Perry,
Issues like this are very difficult to trouble shoot via comments on a youtube channel.
My only advice to you is to compare the good bank sensor output to the bad bank. This should give you an idea of what the sensor's output should be.
Also, I have seen a few reports of counterfeit sensors being sold on on amazon and ebay that don't work or only work for a short time.
My O2 sensor bank 1 doesn't show any voltage. The wiring appears to be good. Should I assume this is bad or are there other ways of checking this? Thanks everyone!
If an O2 sensor doesn't show a voltage, I would scan the car for codes and start to troubleshoot there.
If a code is why you are checking for the voltage, then check for any exhaust cracks/leaks that could allow air into the exhaust system, causing low voltage.
Do a search on RUclips for DannerScanner, he has a few good videos on how to check an O2 sensor.
Good Luck!
Hey got a 2005 Elantra gt I’m using a craftsman obd2 reader and the live graphing says my oxygen sensor voltage at an idle is 1.8-1.9 volts
I suspect you are looking at the wide band / pre-cat / sensor1 voltage.
Most narrow band / sensor 2 / post cat O2 sensors red between 0 - 1 volt.
I don't have work on Hyundai vehicles, so I don't have any experience / knowledge to share. (
Hi, my rear O2 sensor stay at 0.9-0.8 volts all the time, it doesn't switch at all when I accelerate or decelerate, I can assume that it is a faulty sensor or I have a problem with air/fuel mixture? One more thing, my long term fuel trim is around -8%, I checked my first O2 sensor and seems to work properly. A faulty rear O2 sensor can cause this to fuel trim?
When looking at fuel trims, you need to add the short term and the long term to get the total fuel trim. Total fuel trim should be around -10% to about +10%.
If the rear O2 is not switching, it could be the sensor and that might be the easiest and lowest cost fix.
If the engine is still making full power at wide open throttle, more than likely the catalytic converter is ok and not restricting the exhaust flow.
I just came here for an idea on the expected voltages so I can hook up a dual switch with different voltage dividers.
I understand.... keep in mind some ECUs/PCMs will drive the mixture slightly rich/lean and then check the output of the front O2/AFR sensor and compare it to the rear O2 sensor.
This is how they check for a rich/lean bias of the front O2/AFR sensor.
If your ECU/PCM does the above, your voltage divider won't fool it and a P0420/P0430 code may result.
Good video, I'm having an issue where it says my Bank 1 sensor 2 is faulty but showing voltage, but it stays on the same voltage even when you hit the gas 0.450 volts and my short fuel trim from the bank 1 sensor 2 is showing no reading, do you know the problem??
Thanks.
Sounds like your fuel system is running in "open loop" due to a fault.
From what you describe, it sounds like the rear o2 sensor is not functioning properly. (assuming there are no exhaust leaks or cracks)
Hi there
My car’s o2 rear sensor reads 0.7 v at idle or when engine runs on 2000 rpm and reach up to 0.8 v or 0.9 v then come back again to 0.7 v ( mlf on p0420) I replaced the whole catalytic converter
What should i do?
Thank you
As shown in the video, the rear o2 sensor (narrow band type) should have a voltage swing from about 0.1v when lean to about 0.9 volts when rich.
I would check for exhaust leaks (cracks, broken gaskets) and since you have replaced that converters, you may want to test the rear o2 sensor or replace it.
Should I need to replace O2 sensor 2 immediately?
It's not working on my chevy beat or spark.
If you don't have a check engine light, everything is fine.
Maybe your Beat and Spark don't have a traditional narrow band O2 sensor that is shown in the video. I am not familiar with them, so I don't know if there is an issue or not.
My car has 83 000km mileage, but was driven mainly in the city. My scanner shows mostly values minus 100% fuel trim and about o2 sensor voltage around 0.6V with very small +/- deltas. I'm wondering what's wrong - is it the catalyst that produces rich mixture and the o2 sensor tells the ECU to make the mixture lean (minus 100% fuel trim) or it is a problem in the o2 sensor which is stalled at that -100% position. Is it the chicken or the egg first :) There are no error codes. The only symptoms are rough idle on a cold start and engine shaking which is fine after 20s. OBD emission tests says that both catalyst and o2 sensor are not passing the test, but maybe only 1 of them could be the real problem?
Hey lyubo bg,
Sorry for the delay in getting back to you. I usually reply to comments everyday but sometimes my full time job doesn't allow it :)
What is the year, make, model, and engine size of your car? (it helps with the research)
With the engine filly warmed up, what are your "long term % fuel trim" and "short term % fuel trim"
These are the fuel trims from the o2 sensor BEFORE the catalytic convert AND they determine how much fuel is injected into the engine.
They are typically listed before the "Short term fuel trim (Bank 1, Sensor 2)" and are in the range of about -15% to +15% for a proper running engine.
The catalyst doesn't produce the rich mixture, the engine's air to fuel ratio does. The job of the catalyst is to clean up the exhaust gases.
If your rear O2 sensor is stuck at 0.6 volts, it is reporting a rich (too much fuel, or not enough air) condition. BUT it could also mean the sensor is not functioning properly.
The app OBDFusion (about $14) has a really nice and comprehensive diagnostic report.
It displays all the OBD data that is available in an organized format.
(I have no financial connection to OBDFusion, I just like their app a lot!)
It would be very helpful to me if you could run the diagnostic report and I can give you an email address (or you could post it here) to send it.
Then I would have all the data I need so we can work through this together.
@@OBD4Everyone - hi, thanks for the reply, yes, I wrote it incorrectly, the catalyst doesn't produce rich mixture, but the engine, I meant if the catalyst is not doing it's job and thus leaving a lot of carbon monoxide ..
Anyway - the problem is solved in both o2 sensor and catalyst. I bought the car recently (renault clio 1.6 16V) from a woman owner, that has driven it mostly in the city. I took it on the highway with WOT (wide open throttle) and and after 300km both the o2 sensor and the catalyst are reporting "emission tests passed".
And the most happy news is that my engine idle is not rough anymore. Before the RPMs were getting to low and now everything's fine :)
Hey lyubo bg,
Well done!
What you did is sometimes called the "Italian Tune Up"
(Some of the Italian super cars don't run well if they are driven slowly. The fix was a good run on the highway)
With the recent Cobb updates for WRXs I was looking to get tuned but worried about having the dreaded P0420 code as I bought an aftermarket Jpipe with a cat but not sure of if it will throw a code. I was told that it shouldn’t cause the code but if it does the rear 02 spacer should solve it. However several people have been telling me that the rear 02 sensor is used for long term fuel trims and other things that are needed. Based on the videos and others I’m not sure they are right. So my question am I able to use a 02 spacer without effecting anything like fuel trims or anything else?
That is a very difficult question to answer. Each manufacture has their own strategies that are used for fuel control and verification.
I do believe Ford, BMW and Nissan/Infiniti do use the rear O2 sensor for fuel calculations and for checking for an air/fuel sensor (sensor 1) lean rich bias.
For your specific application, the only way to find out is to try it, and see what happens.
@@OBD4Everyone Thanks for the reply. Yes it’s something that has to be expiremented with. I recently put my Jpipe on and it gave no codes so I am fine but I do know that it’s become a huge discussion in the Subaru world.
I get code p0137. I replaced the o2 sensor but the code comes back. I get .025-.035 v at idle but it runs normally while driving .3 -.8v . Would this likely be from an exhaust leak
Hey Wolfy1091,
An exhaust leak absolutely could cause this, as the extra oxygen would lower the rear o2 sensor output voltage.
It could also be from an intake air leak.
You might want to look at your fuel trims at idle. Generally the short term + long term should equal somewhere between -10 to +10. If your fuel trim is positive, the engine control module is added fuel for a perceived lean condition.
Vacuum / intake leaks have a large impact on fuelling when at idle, due to the high pressure difference. At higher engine speeds (> 2,000 RPM) an intake leak has almost no effect on engine fuelling.
Also, with a warmed up engine, the rear O2 sensor should still switch between lean (~ 0.1 volts) and rich (~0.8 volts)
Using graphing function, does your rear o2 switch at idle?
What are your fuel trims?
If you car has a mass air flow sensor (MAF), when was it cleaned last? Dirt build up on it can lower the MAF voltage output at low engine speeds, and over increase at higher engine speeds.
Can you help? I’ve had an issue and can’t diagnose it. I have q bmw 550 and have bad exhaust smell, no check engine, running rich. I replaced upstream sensors and no improvement. Recently got a code P114B and P2196. Short term fuel trim bank 1 is -.80, B1S2 is .81V, short term fuel from bank 2 is -15.61, B2S2 is .14V. Could this be bad downstream? My mechanic says downstream don’t play a role in regulating air/fuel, I’m in a third world country so can’t rely on this info. Any suggestions will help. Thank you
Hey Kevin Kalil,
I will do my best to help :)
I know for a fact that on some vehicles the post cat O2 sensor IS used to check for a rich/lean bias of the front sensor. My 2001 BMW 530 does this, so I think yours might too, but I don't know for certain.
Also on some BMWs (like my 530) it is very easy to mix up the precat o2 sensors connectors. This will cause one bank to be very rich and the other to be very lean.
Sorry but that's all I have. Good luck and let me know what the fix is.
My rear O2 sensor stays at 0,7 volts, I t does not go down much for sometime, I am running lean with +25% fuel trims
I wonder if your rear O2 sensor has failed or maybe there is a crack in the exhaust letting in air/oxygen stopping it from reading rich.
In 2:48, did the author really meant to say "when the switching frequency becomes too low" or "when the switching frequency becomes too high?" The image on the screen is correct: Sensor 2 (dowstream) high switching frequency is not good, but he might have said the opposite.
I have a P0430 code, and Sensor 2 (downstream) is switching faster than Sensor 1 (upstream), so I suspect my CAT is bad.
You have to be careful when comparing the switching frequency of the pre-cat sensor 1 to post-cat sensor 2.
For about the last 10-15 years on most vehicles, the pre-cat O2 sensor is now a "wide band" air/fuel sensor. It will output a steady voltage depending on the actual air/fuel ratio. For example, on my vehicle, if the air fuel ratio is 14.7:1, the output voltage is around 2.2 volts.
Your code is for bank 2. If you haven't already, you can compare the pre/post cat O2 sensor voltage of bank 1.
If your catalytic converter is functioning as expected, and your vehicle has the older style "narrow band" (output voltage from 0.1v (lean) to 0.9v (rich)) the rear O2 sensor should be switching slower then the front O2 sensor. If not, the engine computer will declare a P0420 Bank 1 / P0430 bank 2.
Great video! I purchased a car with the Cat removed. It has a strait pipe with no 02 censor. Check engine light is on, but the car seems to be running fine. Should i try to reinstall the 02 into the exaust, or what are your thoughts? Thank you
Your best bet is to scan it for codes to understand what is going on.
My guess is a P0420/P0430 is the cats have been removed...
@@OBD4Everyone "My guess is a P0420/P0430 is [setting the check engine light] BECAUSE the catalytic converter isn't there."
Federal law (State laws, too?) require that emissions equipment (cats, etc,) be in place and operational. Smog isn't nice.
Did some graphing on a vehicle with a slight hunting idle, lambda 1 shows oscillating between 0.1v to 0.9v, where as lambda 2 sits at a constant 1.2v flat, that a sign of a faulty lambda 2 or more likely a vac leak?
Without knowing any details of the year, model or engine here is my best guess...
If you have a narrow band rear O2 sensor and it is at 1.2 volts, the o2 sensor or wiring is bad. Typical max output voltage is up to about 1.0 volts. If you have a wide-band rear O2, I don't know what it should be as I have no details on what vehicle it is.
An unsteady idle is often caused by a dirty throttle body. As they get dirty from oil/air vapour in the intake from the PCV system, they loose the ability to provide small adjustments that are required for a stable idle.
@@OBD4Everyone it's a 2008 1.6 Zafira, will be narrow band on the lambdas. The rear lambda figure is around 1.2v fixed, whilst lambda 1 is moving between 0.1v and 0.9v. wasn't sure if lambda 2 was put causing the fueling to be constantly adjusting knocking the idle out also
I don't have any documentation on your vehicle.
Since the rear O2 sensors are narrow band, max voltage should be 1v... and since you have 1.2 volts I would verify the o2 sensor wiring and if that tests out ok, replace the rear o2 sensor.
I can't find answers anywhere. My O2 downstreams, even the new ones, are stuck around .7-.8V and barely respond to any engine reving, or driving. That should be rich right? Well, the LTFT are in positive double digits, indicating lean? It's both rich and lean??
Interesting...
When looking at fuel trims you need to add the short term and the long term to get the total fuel trim. In some cases the LTFT can be +20 so you might think there is an issue.. BUT if the STFT is -12, over all everything if good.
What is your total fuel trim for each bank, if you have 2 banks?
Generally the total fuel trim should be around -10 to +10 for a healthy engine.
It is also useful to graph the pre-cat sensor (S1) and the post cat o2 sensor (S2) as you can verify the sensors both show rich under full throttle acceleration, and when the throttle closes, they should show lean.
Questions for you:
What year/make/model is your vehicle?
Do you have a trouble code?
Does the engine run normally and make full power?
@@OBD4Everyone No, the short terms are sitting at 0, with +-3 tolerance. I find a leak at one of the hoses, and my LTFT seemed to drop to 5 and 7% yesterday respectively, but today they were now at 16%, with the STFT around 0 again. :(
Yes I have two banks. So the totals for example on today's numbers would be 16, 16+0. The worse case I had was 21%. Sometimes it decreases when rev, so I assumed vacuum leak. Other times, it goes up.
I actually got confused with the data. The downstream for this toyota engine is supposed to be flatline (apparently). They aren't "stuck". But, ideally it should be around .45V, not higher or lower (although again that's ideally). The A/F sensors on the upstream are both working fine, sine wave behavior around 3.3V.
2007 Lexus ES350. No trouble codes. I had a brief misfire code P0300 weeks back when I was having trouble starting (I'm apparently having alternator problems according to autozone today). Though I never noticed the check engine light, I only saw apparently it was a stored code that showed up 2 days prior to me checking with the scanner. Never returned after I cleared the code.
Engine seems to be rough? It's my first car so its hard for me to judge on what exactly is normal, but it vibrates a bit at idle in the wheel, pedal. The phone holder on my windshield you can see vibrate.
Could a failing alternator be the reason for a lean condition? Maybe poor power delivery to the fuel pump? I have a thread here I made on a toyota forum if you want to check it out, it has pics of the data I have from my scanner.
www.toyotanation.com/threads/can-someone-clarify-what-is-normal-o2-downstream-behavior.1716453/
Great video...would the O2 sensers be able to show block Cat or even a clogged air filter...or would the MAF sensor be of help...
Hey Geoff,
Good question!
If the catalytic converts are plugged, the rear O2 sensor will switch from lean to rich VERY slow or not at all. (Assuming there is no exhaust leak)
The mass air flow (MAF) sensor would give you a better indication of a blocked catalytic converter. If the exhaust gases can't exit the engine, the intake air flow will be greatly reduced.
BUT how do you know what the MAF value should be?
If you have data logged the MAF when the engine was in good condition, you know what the MAF value should be.
-OR-
You can calculate an approximate grams/second flow, based on engine displacement and RPM.
For example:
•A 225 cubic inch motor at 7,000 RPM
• WITH the throttle wide open (max flow)
grams/second = (engine size in cubic inches x RPM ) / 6095
g/s = 225 x 7000 / 6095
g/s = 258
Now, this simple formula is only a rough guide, but it should give you an idea of what it should be.
** Keep in mind, this simple formula doesn't factor in the change in air density due to temperature and elevation **
@@OBD4Everyone my car is running well...got 2 g/s....so if it had clogged cat this would read around 1 g/s. If air cleaner was restricted would this also be less than 2 g/s
Hey Geoff,
I assume the 2 g/s was at idle, and that your engine size is about 2 litres.
Since the air flow at idle is very low, that isn't a good test to determine if you catalytic converter is plugged or partly plugged. However, in your example, I agree with your conclusion.
A better test is to monitor the MAF output while accelerating at full throttle in a safe area. This will create much more air flow through your engine and exhaust. Then you can compare the amount of flow to the calculated amount shown above. They should be fairly close.
With your car running well, now is the time to do the wide open throttle test to determine maximum MAF air flow.
In a few years you can re-do the test and see if the maximum value is lower. If it is, something it reducing the engine air flow.
Have fun testing :)
@@OBD4Everyone cheers for the reply..look forward to your next video....
Now here's a mystery. I have a 2014 Mitsubishi Mirage that's getting a P2096 code for "post catalyst fuel trim system lean." However, the upstream wideband O2 sensor shows it's slightly rich the majority of the time, and the rear sensor voltage fluctuates between .54-.88, also indicating it's rich. But somehow, I keep getting P2096 lean code popping up repeatedly. This doesn't make any sense.
Don't just focus on the voltage level, also have a look at the rear O2 switching frequency. (low to high and high to low)
What are your fuel trims at a hot idle and on the highway doing 50 Mph?
@@OBD4Everyone Unfortunately, my UltraGauge doesn't map out the switching voltage, so I'd need a better scan tool for that. LTFT has gone between 3-5 at idle and 5-7 at a highway cruise. I replaced the rear O2 sensor (it has 115k miles and I replaced the front already), and the code was still showing up with the new senor. I pulled the battery to reset the memory and have it re-learn the fuel trim, and had a P0421 come up for the catalyst underperforming during warm-up during the first day. I reset the code, and none of them have come back since.
Could you use the mass air flow rate instead of the accelerator position? Or does the accelerator position provide better specificity for sensor behavior?
Hey wanderingazn,
The mass air flow rate is also a good indication of engine load.
You could also use calculated engine load, absolute engine load, MAP (intake manifold vacuum), and relative throttle position.
It is just nice to have something capture engine load, to help you understand what the O2 sensors are doing.
Have fun :)
@@OBD4Everyone Thanks! I've been graphing that as my Graph 2 since my vehicle only has one downstream sensor. Wanted to make sure I had a good marker for my engine load!
Hey wanderingazn,
Anytime :) Have fun!
Hi, I have seen several Videos that uses an O2 spacer to remove the P0420 and P0430 codes. What does this spacer do? Just for my knowledge.
I have a 2021 Hyundai Palisade V6 and the above 2 codes are being thrown. How is it possible that both Banks come faulty at the same time? The car is only 10,000 Kms driven
I would assume you car is still under warranty, so take it in and let the dealer fix it. I suspect there is nothing wrong with your car, as it is too new, and it needs a engine computer update.
The P0420/0430 codes check the efficiency of the catalytic converter. If the cat is working, the signal from the rear (after cat) sensor has a lower switching frequency than the sensor before the cat.
The O2 spacer try to trick the engine computer by removing the sensor from the direct exhaust flow. HOWEVER new engine computers seem to have the ability to detect this and the P0420/0430 code stays.
@@OBD4Everyone its not under warranty since i have imported it.
Hi, very nice video! Thanks a lot for it :)
I have two very old (maybe dead) O2 sensors downstream and I will try to use your method to test them. So far I noticed they always stay around 0.1V.
I have just replaced my pre-cat O2 sensors and they appear to be fine but the car still has very very poor gas mileage. I have 20% LTFT Bank1 (new plugs) and 25% LTFT Bank2 (old plugs).
Can the downstream sensors have any impact on the current fuel consumption? :( everyone said so far they don't help much so I kind of ignored them..
Following codes appear: P207F-001 Left O2 sensor, before TWC - Aging, correction variable exceeded: Delay time too long (PO2098) /// P200C-001 Right O2 sensor, before TWC - Aging, correction variable exceeded: Delay time too long (P2096) /// Right O2 sensor, after TWC - Level is above applicable threshold (P1999).
Have no idea what to try next cause I replaced a looot of stuff :( Maybe you can help. Thanks a bunch!
Hey andybgdn,
Please tell me the year, make, model and size of engine so I can do some research.
On some engines, the rear (post cat) O2 sensors ARE used in the fueling calculations as well as checking for catalytic converter efficiency.
Try this: unplugging the rear O2 sensors (you will get some codes for O2 sensors heater) go for a drive, and see if the fuel trims change.
I assume you have OBD Fusion... With the engine fully warmed up, run, save and email to yourself OBD Fusion's diagnostic report. Depending on which car you have, there can be some VERY helpful information in the mode $01 and mode $06 sections.
Hi mate,
I have a 2003 mercedes clk 240 w209 m112 engine v6 2.6l with 4 o2 and 4 cats!
If I unplug the downstream sensors I get open loop running and no Ltft is being changed at all! System is running in open loop 24/7. After I connect them it goes back to CL.
I have iCarsoft MB II Mercedes to diagnose but it s not very pro when it comes to graphs.
Hey andybgdn,
If I found the correct documentation, your iCarsoft MB II has an "O2 Sensor Test" under diagnostics (page 14 in the manual) . It should tell you everything you need to know about your O2 sensors. (I suspect this is the OBD standard Mode $05 which is good, as it gives a lot of information to help you)
I would also try graphing the front and rear O2 sensors. I suspect the data update is slow, but it should still be usable:
(1) Drive at a steady speed, the front O2 should be switching frequent between about .2 volts to about .8 volts. The rear should have the same voltage, but switching much slower.
(2) At wide open throttle, the front and rear O2 sensors should both be more than .8 volts.
(3) Then lift your foot off the gas pedal, the voltage of both O2 sensors should be less than about .2 volts.
If the O2 sensors have more than about 60,000 miles / 100,000 KM they are most likely due for replacement. Check the maintenance manual to see what their schedule is.
Also, dirt buildup on the mass air flow sensor (MAF) can cause it to under/over report the air flow and that can cause false lean/rich issues.
If you haven't cleaned it recently, you may want to:mbworld.org/forums/c-class-w203/268854-m112-maf-sensor-replace-cleaning.html
Good luck and please let me know your results.
Thank you for the amazing video. I ordered my elm327 adapter and will be downloading OBD Fusion for Windows. I am getting a P0420 on my 4 cylinder 2006 tacoma with only 95,000 miles and am trying to confirm that it is the "very expensive" Cat. This truck has an A/F sensor on the front of the Cat. Is the procedure you present in this video the best way confirm that it is indeed the Cat? Thanks again.
I think so... The video shows the typical operation of a working catalytic converter and the resulting narrow band rear o2 sensor voltages for different operating conditions.
Assuming there are no exhaust leaks, your rear O2 sensor voltage should change in a similar way.
A P0420 looks at the switching frequency and delay from sensor 1 to sensor 2.
You could data log your front A/F sensor and rear O2 voltage, while driving around for 10+ minutes to ensure they are warmed up and operational.
Then find a safe sport and do a few seconds or wide open throttle, then lift off the throttle come to a stop and let the engine idle for 5ish minutes.
During wide open throttle, your A/F ratio should be somewhere in the 12s and the rear O2 at > 0.9 volts. When you lift off the throttle, the A/F ratio should increase to 20+ and rear O2 voltage should drop to < 0.1 v.
You also need to look at the time delay between the front A/F ratio going rich/lean compared to the rear O2 sensor. This shows the oxygen storage capacity of the catalytic converter and is what the P0420 tests for.
From tests I have done on my G37 there is about a 1 second delay between the front A/F sensor going lean and the rear O2 sensor going lean / < 0.1v.
I would expect your Toyota to be similar, but I don't have any first hand experience on that.
The last thing you can do, it to look at the mode $06 data in OBD Fusion's diagnostic report. There is a "Catalyst Monitor" that shows the condition of your catalytic converter.
You may want to read this to learn more:
www.motor.com/wp-content/uploads/AdvancedMode06Diagnostics-March2007.pdf
Good luck!
@@OBD4Everyone Thank you so much for taking the time to help. I will let you know how it turns out.
@@wayneferrell9646 it could also be a leak. Good way to find one is with a shop vac and soapy water. Blow air through the muffler side and spray soapy water on all of the exhaust system from beginning to end, if there's a leak bubbles will start spewing out.
If all is good try using CATACLEAN at least 2 times when you fuel up it's pretty good. Sometimes using cheap gas can build up crud in the system that'll help clean it.
I did all these steps including replacing O2 sensors (they were overdue at 300k miles) and i got rid of the code and completed all drive cycles for smog. Toyotas are pretty sturdy so i wouldn't replace the Cat right away
@@wayneferrell9646 how it turn out ?
Quick question because I’m curious if it’s my cats, or o2 sensors. So at a steady 50 mph, bank 2 fluctuates between .01-.8 while bank 1 stays at .72-.73 and doesn’t move. What could I be dealing with here?
Mind you. This is in a 2013 Honda Pilot EX
I don't have any first hand experience on Toyotas, but it looks like bank 1 has a problem... either a crack in the exhaust, bad O2 sensor or maybe bad cat.
Generally both bank 1 and 2 should have similar voltages for the rear O2 sensor.
Hi, my car has a po420 code and my o2 bank one sensor is negative so - 0.03. Is this faulty sensor. Thanks
Based on what you have told me, I would think so. An O2 sensor's voltage range is generally between 0.1v and 0.9 volts.
So, the rear O2 sensor reading mostly 0.000 is bad? It reads 0.000 to 0.035. I just cleaned out the cat, as it was a bit dirty. It was giving the code for poor catalist system. 2013 Toyota Echo.
Correct. The rear O2 sensor voltage levels should swing between around 0.1 volts (lean / lots of O2) or 0.9 volts (rich, very little O2)
I would carefully examine the exhaust for any cracks, holes or leaks. If extra air gets into the exhaust, the sensor will show a low voltage.
It is possible the O2 sensor needs to be replaced.
@@OBD4Everyone Thank you.
So if mines switching rapidly between rich and lean at idle is then it's no good right?
Unfortunately my motor is a 4 cylinder so I don't have another sensor on the car to compare it too.
Without knowing the details (car, engine, etc) the rear o2 sensor should switch slowly over 3-5 or more seconds at idle. If the rear O2 is switching quicker than that, it does point to a low catalyst efficiency.
@@OBD4Everyone so would that be an O2 fix or cat fix?
have a 2002 jeep wrangler, just replaced bank2 sensor 2 and getting PO158 and P0161 codes, live data shows a constant 1.000 volts. Could it be the new sensor is defective or is it likely a shorted wire?
Hard to say. Remote troubleshooting through RUclips comments is tricky.
It could be a bad O2 sensor, bad PCM, or fuelling issue (too rich)
You (or a good tech) will have to work through the steps to troubleshoot this.
Good luck!
it's clear P0161 your prob is with the heater of the 02 sensor bank 1
see youtube how to test the heater
My 02 sensors they stay around .7 is that good or bad? They don’t go up and down
Generally, under full engine load, they should read rich or around 0.9 volts, then when you lift off the throttle, they should read lean or about 0.1 volts.
If you don't have a check engine light, then there is no need to worry.
Best video i ever seen...i want ask you a question please....l have audi a4 2005 ...l have code 17526 sensor heat open circuit b1s2 ...p1118 .Does it affect on engine behavior on idle ..cuz l have unstable rpm when i press gas pedal
Hey Mahmoud Hammoudeh,
Best video? Wow, thanks!
When the O2 sensor heater is not working it won't be at proper operating temperature and provide the proper feedback.
In some cases the rear O2 sensor's signal can be used to check for any lean or rich bias on the front O2 sensor. This might be what is going on with your car.
You could try disconnecting the rear or sensor and see if the issue goes away.
If you have an OBD scanner, check your fuel trims at idle with the engine fully warmed up. Add the short term and the long term fuel trim for each bank (if you have more than 1) to get your total fuel trim.
The total fuel trim should be around -10% to +10%.
If outside of this range, your rear O2 sensor might be used to change the fuelling.
To verify the issue, you will need a wiring diagram to find out which wires at the O2 sensor are for the heater. With a cold engine (ensures o2 sensor heater is on) and switch the ignition on and check for battery voltage at the heater wires. (should be 12-14 volts)
Also you can check the resistance of the O2 sensor heater, but you will need to find the specs online.
@@OBD4Everyone
Thanks alot for your quick response ....i appreciate this video cuz there is no videos talk about post oxygen sensor ..after obd check i found downstream sensor is deactivated from ecu ...do you think the problem its sensor fault?
Hey Mahmoud Hammoudeh,
Those most likely cause is the wiring or the O2 sensor.
It would be best to check for the heater voltage at the O2 sensor to verify the fault, but if you don't have the tools for that, you could try changing the rear O2 sensor.
Why do you think "downstream sensor is deactivated from ecu"?
Hi, i have P0036 code heater control circuit on bank 1 sensor 2, i think this code showed up cause recently i have leaking coolant on cylinder 1 and 2 so i bring it to mechanic for headgasket replacement, now its running fine but when i check the misfire monitoring data , sometimes each cylinder has 1 misfire, but not all four of them together, every day i have 2 total misfire on random cylinder, it could be the faulty rear o2 sensor?
A very low number of misfires is nothing to worry about.
If the car has a manual transmission and the engine RPM is too low when you let the clutch out too quickly, that can cause a misfire to be detected.
Going forward, just keep an eye on them and see if the number of misfires increases. Good luck!
@@OBD4Everyone wow thanks for the response my car is AT. This reply gives me peace of mind really, thank you and more power to your channel 👍🏻👍🏻👍🏻
My 2005 mustang gt was diagnosed for clogged cats so i went ahead and replaced with used ones that had about 40k miles on them. I now have an exhaust leak and the car is still running weak. No check engine light. I took it to ford and the downstream 02 sensors were reading low and would move up and down. They explained that the readings werent steady therefore they said the catalytics are plugged! Im hesitant to drop 800 dollars for new cats what can i do at this point?
Hey Exile,
The rear o2 sensors should switch at a frequency less than the front for proper catalytic converter operation.
Maybe the dealer meant to say... "they need to switch slower" not "they shouldn't switch"
If the front and rear o2 sensors are switching at about the same frequency, the catalytic converter is not functioning properly.
If you can find a shop that can do a "5 Gas Exhaust Analysis" that will tell you the condition of your catalytic converter:
www.walkerexhaust.com/support/tech-tips/five-gas-diagnostic-chart.html
Here is a good article:
www.motor.com/magazinepdfs/051998_09.pdf
Another option is to remove the pre-cat o2 sensor and screw in a pressure sensor to record the exhaust pressure before the catalytic converter.
I wish you luck with whatever you decide to do. :)
@@OBD4Everyone thanks im already looking into checking back pressure greatly appreciate it!
Good luck!
Thinking I could need a cat. But I feel like I need fuel injectors or to check distributors and line/pump. Running lean on my idle. Either I’m not getting air out quick enough or I’m not getting enough fuel in.
Hey Dps Heals,
Why do you think you are running lean?
What year, make & model do you have?
06 Honda Civic 1.8 sohc:
I think I was wrong. From the info you told me everything seems correct. The car is fine. I’m just now getting into cars after 25 years! (Yea, I know) and I’m playing around with it. I put an intake on then immediately would get a boggy/ stall from idle to acceleration. So I was trying to diagnose it. Seems like it’s getting better though... I thought maybe an o2 sensor wasn’t reading right or maybe it’s just too much intake and not enough exhaust. Maybe I need new fuel injectors... I’m just having fun with this new hobby. Not in any crisis with the car but any info on anything would help. Btw those scanners are awesome! And cars are like a puzzle. I love it!
Thanks for taking your time to reply I appreciate it!!
Hey Dps Heals,
No problem.
Sounds like you have found a new and fun hobby :)
So if the second sensor is staying around .6-.7 volts and not switching at all during acceleration or revs I would assume the sensor is bad?
Yes, or there is an exhaust leak letting in air and making the O2 sensor read a little leaner.
@@OBD4Everyone Sorry for the late reply but I just found your video (great job by the way!). I thought I was finally 'getting' O2 sensor response until I read your reply to Jalpa. If the downstream sensor is reading constant 0.06 - 0.07 V wouldn't that indicate a rich condition? And wouldn't an exhaust leak create a lean condition? Thanks in advance!
For narrow band, rear O2 sensor, 1.0 volts is rich and 0.0 volts is lean...
So you have a dead sensor or a VERY lean condition.
Hi , very informative video. What software app are you using to plot the graph?
That's the graphing function of OBDFusion that has been edited to speed up the boring parts :)
Great set of videos...will you be doing anymore...Geoff
Hey Geoff,
Absolutely!
Right now the "OBD Scanner Buyer's Guide & Speed Test" is in post production and should be out in a week. 11 different cheap OBD scanners have tested and reviewed.
I have a long list of episodes in the production schedule.
What would you like to see?
So could an exhaust leak affect the readings the downstream 02 sensors are putting out?
Hey Exile,
You bet! If the exhaust system has cracks, air can leak in and the oxygen sensor can react to that. Keep in mind, the exhaust system has pulses of positive and negative pressure. When it is slightly negative, it can pull in additional oxygen. (technically, the higher atmospheric pressure pushes into the lower pressure exhaust through the crack)
@@OBD4Everyone I see! My car was diagnosed for having clogged cats i took it to a shop because my car had really poor acceleration it had no check engine light. So i bought used ones that had according to the guy who sold them 40k miles on them. I went ahead and installed and i noticed a sorta noise when starting cold and when accelerating. For sure i have an exhaust leak and im wondering if perhaps the cats i put in are good and maybe that leak at the flange is causing the performance issue since the downstream 02 sensors are reading low and the high not steady.
My rear sensor 2 bank 2 under acceleration goes to 0.070 while my rear bank 1 sensor 2 fluctuates to like .045 and when I let go of the gas it goes to 0.09(bank 1 sensor 2) while 0.060 (bank 2 sensor 2). Does that mean my sensor 2 bank 2 is bad? Thanks
Check on Google about proper readings.
(I am assuming your vehicle has the typical "narrow band" rear O2 sensors.)
Both of your rear O2 sensor's voltage output are too low. Under hard acceleration (wide open throttle) they should be around 0.9 volts (rich), and when you lift off the throttle after the hard acceleration, they should drop to about 0.1 volts (lean)
Once the vehicle is warmed up, use OBD Fusion to run a diagnostic report. Under the "Mode $01" section it will show you the supported sensors. I am thinking you might be looking at the wrong Pid (sensor)
For example on my vehicle, I need to look at the "O2 voltage (Bank 1, Sensor 2) and O2 voltage (Bank 2, Sensor 2)"
If you are looking at the correct PID, check you exhaust for any leaks or cracks. This can allow air into the exhaust resulting in the rear O2 sensors reading lean.
Is it normal for the SHRTFTB1S2 (Rear O2 Sensor) to read -100%?. Voltage is good and fluctuates as it should. Only SHRTFTB1S1 seems to have the % fluctuate normally. I have an inline 4 cylinder so only 2 O2 sensors for me. Cheers.
Hey Vince Granville,
I think the "short term fuel trim" for the post cat O2 sensors (SAE PID 0x15) don't tell us anything.
For example, on my G37 it has ALWAYS showed "short term fuel trim for sensor 2 as 99.2% and has not changed in the 5 years I have been collecting data. This is also true on 3 other G37s that I have checked.
So.... if your post cat O2 sensor is around 0.9v when accelerating and 0.1v when off the throttle all is should be good :)
@@OBD4Everyone thanks for the reply. Post cat O2 sensor comes in around 0.750 when accelerating and around 0.175 when idle. Rough figures. So based on that and your reply the short term fuel trim post cat I don't think does anything. Unless of course the cat was maybe bad then it may change accordingly. Thanks. Appreciated.
Anytime :)
Got the dreaded p0420 and 430 codes. The mechanic swears it’s a cheap after market cats causing a cel .The rear 02 sensors are fluctuating and not steady.i don’t think he has checked for any vacuum or exhaust leaks. Could that be throwing off a code ? He wants to use GM cats but the oem cats will cost thousands to put on.
Unfortunately, it seems like some aftermarket cats don't work well enough to keep the PCM/ECU happy.
If your rear O2 sensors are the "narrow band" (outputs 0 - 1 volts) then the rear O2 sensors should be switching, BUT a little slower than the front O2 sensors (assuming they are narrow band as well, and not actual air/fuel ratio sensors)
Great video. My bank 2 sensors read rich...even when my foot is off the accelerator. Engine issue, or cat issue? I am pretty sure the previous owner had the truck on a programmer. I am getting reflashed ASAP. Any thoughts would be appreciated.
Hey J Ath,
I am going to assume you are talking about the rear (or post cat or sensor 2) O2 sensors. If your issue is with the pre-cat O2 sensors let me know and we can troubleshoot that.
A quick test would be to compare the output voltage of the pre-cat O2 sensors to the post-cat O2 sensors. If you have the older style "narrow band" O2 sensors, the pre-cat O2 sensors should be switching from lean to rich much quicker than what the video shows.
Also have a look at your fuel trims. Add the short term and long term to get the TOTAL FUEL trim for each bank. They should be in the range of -10 to +10% if the engine is in good condition.
Going back to a stock tune/flash is a good idea. I would also have a look at the O2 sensors to see if anything is non-stock and that the wiring is good.
My post cats sit at about .700. Is that overly rich?
Hey J Ath,
When at operating temperature and under light engine load or idle, the post catalytic converter O2 sensor should be switching between about 0.1 to 0.9 volts.
If your rear O2 sensor is always at 0.7, I suspect the catalytic converter might of been removed or hallowed out. Since the vehicle had a programmer, I think there is a good chance other modifications might of been done.
Quick check:
With the engine warmed up, the exhaust should not have a strong smell if the catalytic converter is functioning properly.
@@OBD4Everyone Okay, I have been watching the data quite a bit. The B1S1, B2S1, and B2S2 fluctuate. The Down stream sensor does not fluctuate quite as often as the up stream sensors do. However, the B1S2 fluctuates only betwenn 590 and 700. It never goes below 550.... Thoughts? I will also get an occasional 3 cylinder misfire. 5.3 V8.
Hey J Ath,
I am assuming you have a L83 5.2 V8.
I think cylinder 3 is on bank 1, which is the same bank that has the post O2 sensor the doesn't switch down to 0.1 volts. I wonder if there is a connection?
How often does the cylinder 3 misfire happen? It is very bad if raw fuel gets to the catalytic converter, as it can melt the core and partly plug it. This might explain the somewhat static output of B1S2.
To test you could remove both post cat o2 sensors and swap sides (assuming they are interchangeable and have the same cable length).
Then look at the data and see if the signal switches sides.
If they can't be swapped, the next cheapest option would be to replace the B1S2 O2 sensor and look at the data.
I think you also need to track down and fix that cylinder #3 misfire, to maximize the catalytic converter life.
Surely if the cat is doing its job then the post cat sensor shouldn't be reading a rich mixture,your graph was showing pre cat sensor voltages not post cat sensor voltages.The post cat sensor measures oxygen like the pre cat.so if the cat is working the post cat sensor voltage should be at steady voltage about.5v not jumping up and down like the pre- cat sensor.correct me if im wrong.
Hey Terence Wyn Roberts,
The O2 sensor shown is the post catalytic converter sensor, also known as "Sensor 2"
"Sensor 1" is the pre cat O2.
In order for the catalytic converter to function properly the air fuel mixture is constantly switching between slightly rich and slightly lean during light load or crusing. The allows the cat to "re-charge" with oxygen so it can perform its function.
Since this car has a "narrow band" post cat O2 sensor, it response like this:
• When the air/fuel mixture is about 14.7:1, it will output about 0.5 volts
• When the air/fuel mixture is less than 14.7:1 (like 14:1), it had detected a rich condition and the voltage will quickly increase towards about 0.8 volts.
• When the air/fuel mixture is more than 14.7:1 (like 15:1), it had detected a lean condition and the voltage will quickly decrease towards about 0.2 volts.
The pre-cat O2 sensor will switch much quicker than the post cat O2 sensor, but the post cat O2 sensor still switches about every 2-3 seconds.
This is how the ECU (engine computer) determines the health of the catalytic converter.
If the pre-cat and post-cat sensor switch at about the same frequency, the ECU will put up a P0420 "Catalyst System Efficiency Below Threshold", because the cat isn't functioning at the required level.
Does this answer your question?
Thanks for your comment :)
@@OBD4Everyone nice one ,understand now as you explained the slower switching of post cat sensor.Thank you,keep up the good explanations.
Hey Terence Wyn Roberts,
Excellent! I''m glad you agree. :)
You are actually correct. The post cat sensor should indeed produce a very flat graph line. The information in this video is completely wrong. If the upstream mirrors the downstream it means the catalytic converter is no longer working.
Hey Ian King,
For a good catalytic converter the front and rear O2 sensors can be the same in amplitude (voltage), but not frequency.
From Motor Magazine Dec 2011:
"A good rear oxygen sensor reacts to changes in the air/fuel mixture just
like the front sensor does. Creating an artificially rich condition
should promptly push the rear O2 sensor’s signal up to or very close to
.900 volt. An artificially lean condition should promptly drop its
signal down to or very close to .100 volt. Another option is a simple
snap-throttle test. My experience has been that a good rear O2 sensor
will respond predictably to the sudden rich-lean conditions a
snap-throttle test creates.
...
Furthermore, a healthy converter that’s absorbing incoming oxygen
acts like an oxygen dampener or shock absorber. This makes the rear O2
sensor react slower than the front sensor. But a failing, inefficient
converter doesn’t create the dampening effect because it doesn’t absorb
oxygen. Without the “dampener,” the rear O2 sensor reacts abruptly to
incoming oxygen-hence the cycling scope pattern in Fig. 1. It also
spawns the P0420 trouble code."
Full article here: www.motor.com/magazine-summary/foreign-service-3-december-2011/
Hey, just for kicks, why not graph your own rear O2 sensor and see what it does?
Thanks for you comment :)
I have Hyundai Getz old car and most of the time O2 sensor 2 reading shows -7.01mV. Also Error code came up P0133. What could be the issue?
Hey Amila,
Sorry, but I won't be much help... I have never worked on a Hyundai Getz as they were not sold in North America.
@@OBD4Everyone thank you for replying
My downstream o2 sensor is reading 0V, replaced it with 4 o2 sensors and one From a dealership, its Still reading 0. What Could be the problem?
That's really hard to say...
It could be a broken or shorted wire between the ECU/PCM and the O2 sensor, a fault inside the ECU/PCM, or a bad ground... just to name a few items.
@@OBD4Everyone that side that’s faulting had a wire snapped off from the sensor by the driveshaft. I don’t see damage from the plug, but it’s possible that it’s damaged
I have 04 BMW 325xi my question to you is does the rear O2 sensor 2 Downstream is it a flat is it supposed to do the exact same thing as the front ones osculate
Hey Isreal Perez,
It depends on the type of front O2 sensor. I am 99% certain that your car has the older style "narrow band" O2 sensor.
Newer cars use an Air/Fuel sensor that measures the actual air/fuel ratio. The older "narrow band" sensors basically switch between lean (>14.7 air/fuel ratio) and rich (< 14.7 AFR)
For the narrow band front O2 sensors, if they have the same waveform (cycle frequency and amplitude as the rear) the catalytic converter function is low.
For a proper running system, when fully warmed up and at low engine load (idle or cruising at a steady speed) the front O2 sensor should switch between about 0.1 and 0.9 volts and the front should switch quicker than the rear O2 sensor. (also around 0.1 and 0.9 volts)
@@OBD4Everyone on last question I've found b1s1 wires fuse together the plastic melted off so I've separated them and taped it off would this kill the ecm that reads p0171 b1s1
Hey Isreal Perez,
I think your ecm (or DME in BMW speak) is fine.
Since the trouble code is NOT for an o2 heater circuit, the wires that shorted together are for the input to the DME. That's good.
If you plan on keeping the car, you might want to get a new O2 sensor and try to figure out why the wires melted. That's not normal.
Good Luck!
So on idle the downward sensor should have a steady line but during higher rpms should fluctuate.?! I think I need a new cat. Do for an upgrade anyway. I put an intake on and now I’m assuming I’m not getting the air out fast enough. Thinking it could start with worn out valves to the manifold/cat/ exhaust system.
@@dpsheals When the rear O2 sensor is at operating temperature, during idle it should still move between lean (
Thinking the catalytic converter was a response to carburetors and early EFI and now that fuel metering is so advanced its now just a restriction that demands you waste fuel to keep it warm.
If you are referring to the typical "cold start strategy" to get the cat up to temp, it only lasts for about 60 seconds and greatly reduce emissions as fast as possible.
are you saying the cat never falls out of operating temp?
Hey OBD4Everyone, Great video! Question for you...I have a 2007 Dodge Charger RT that's coming up with the P0420 abd P0430 codes. Checked the graphs you described in the video and it falls in line to your comment that this will occur if the downstream sensors don't shift back and forth as often as they should. Generally, mine are staying fairly flat at 0.2V or 0.8V, but rarely flips (takes more than a few minutes). Is that a sign of bad cats? I'm getting it for both Bank 1 and 2. Upstream sensors are oscillating nicely as they should. I've been quoted at a shop for 2 bad cats, but another shop said exhaust manifold leak. What should I do next? TIA!
Hey Daniel Kelepouris,
Interesting question... I think you need to ensure that you don't have an exhaust leak. Any extra oxygen in the exhaust may decrease the rear O2 switching, setting off a P0420/P0430.
If both codes popped up at the same time, I wouldn't think both cats would fail at the same time, but a crack in the exhaust or split flex pipe might cause that.
Did one code pop up first? What is the mileage on the car and the rear O2 sensors?
One more thing... thank for your kind words on the video :)
@@OBD4Everyone
Yes, both codes popped up at the same time which was odd that the diagnosis from one shop was 2 bad cats. Didn't feel right. Car has original sensors and it's at 98K miles. Soon after those codes, I had an oil pressure sensor code pop up and that was replaced. Just including that in case that's any indication of what that could be.
Also, that same shop has recommended Cataclean as a potential fix or at least temporary fix to pass a smog in the near future. What's your take on that product? Seems like a work around, but not going to solve anything.
Thanks again.
@@OBD4Everyone
Also, you mention an exhaust leak could decrease the rear 02 switching...would that require that both manifolds to have a leak? If so, wouldn't that also be odd timing for both to trigger at the same time? Mine has the HEMI 5.7L. Thanks.
Hey Daniel Kelepouris,
Since you have 2 banks, it *might* be possible for air to enter on side of the engine that has a leak and move to the other side of the engine.
We tend to think of the exhaust flow as only moving in one direction... BUT it doesn't, there are multiple pressure pulses bouncing around inside the exhaust and this could allow extra O2 from one side to migrate over to the other.
As you can imagine, it is difficult to troubleshoot problems from a keyboard, but if I were you, I would fix all exhaust leaks first, THEN see if the rear O2 sensors output is any different.
@@OBD4Everyone Thanks so much! Appreciate the help
My pre cat AFR 02 is out am getting 0.8 ~0.9 volts with all drive cycles at my rear 02 sensor.
Normal?
Nope... That indicates a constant rich reading (assuming you have an O2 sensor and not an air/fuel ratio sensor)
If you accelerate hard and then lift off the gas, the front o2 should read lean or about
@@OBD4Everyone No, I don't have check engine light or code. I have Afr sensor, and at car start up when you monitor the fuel trim stft via obd is always stuck at 25% whiles ltft is 9.36%. Changed it to multiple afr sensors still the same, the shocking thing is when you take out the afr sensor from the hole and put it outside and start the car as sensor harness is connected still the same values as mentioned above is the one which is registering. What can be really be the cause ? Any direction I should go?
Hard to answer.... I don't know anything about what vehicle you have, so I am 99% in the dark.
If you don't have a check engine light and the vehicle drives fine, maybe there isn't an issue?
Great video buddy. Quick question, both O2 sensors have a spikeand drop voltage. Bad cat?
Sorry, but I don't know what you mean by " spikeand drop voltage"
@@OBD4Everyone like for examples the top sensor if you graph it, its suppose to graph up and down. But the downstream sensor also does it.
Generally, if the front O2 sensor and the rear O2 sensor have about the same waveform, the oxygen storage capacity of the catalytic converter is very low and may need replacement.
@@OBD4Everyone in my case I have a 2013 Cruze 1.8 non turbo. So I have a p0171 running rich code. No vacuum leaks, no bad sensors and good spark. Car wants to stall when coming to a stop and idles rough. I DID notice however the downstream O2 sensor has the same voltage as the upstream one. But no cat converter codes. Maybe the bad cat is causing the stalling?
I would datalog or monitor the fuel trims while driving the car.
I wouldn't think a "running rich" code would be caused by a bad cat.
Cleaning your MAF (if you have one) might help. When they get dirty they over report the air flow at low engine speed and under report it at higher engine speed.
You can't compare upstream voltage to downstream voltage UNLESS you know that the upstream is actually a "narrow band" O2. Many cars made in the last 10-15 years use an "air/fuel" sensor and have a VERY different voltage output.
Stupid question I have looked all over for odb fusion for PC and cannot find any legitimate sites. Who make this software or is this strictly for phones/tablets?
OBDFusion for the PC is called OBDWiz. Here is an episode on it:
Tested: OBDLink SX + OBDWiz, the fast USB Scanner: OBD4Everyone E21
ruclips.net/video/Uw4jxQQjMag/видео.html
what if i dont use o2 sensor rear...check engine be light?
Correct!
Its job it to check for proper catalytic converter function.
If the engine computer (ECU/PCM/DME) doesn't see the O2 sensor, it will turn on the check engine light.
Used this information this evening. Thanks! One question - What would you expect to see if a test pipe was installed in place of a cat? Working on an offroad rig and I am getting very different readings between 1 and 2; I'd have expected them to be closer, essentially reading the same exhaust, unaltered. Correct?
Hey Brian Kimmitt,
I need more information.
What engine are you working on?
What type of sensor is before the cat:
Is it an O2 sensor (narrow band) or a Air/Fuel Sensor (wide band)?
If I assume O2 sensor 1 is an old style narrow band and you are not using anti-foulers / extenders on the rear O2 sensor, they should be very similar with a test pipe.
But if sensor 1 is an Air/Fuel sensor, the voltages will be VERY different.
What reading are you getting?
I have a 08 traverse with a rich condition on both banks, front 02 sensors are switching normal to rich lean, but the rear sensors are reading only from like 430mv to 500mv,
and the rear sensors never goes into close loop, do you think the 02 sensors are ok? or it should be reading higher and lower than that?
Hey 100ega,
According to Autozone parts look up, your front and rear O2 sensors are "narrowband" and the output voltage range should be .9 volts (rich)
Questons:
• Why do you think the engine is running rich?
• What is the rear O2 sensor voltage at wide open throttle? It should be > 0.8 volts
• What is the rear O2 sensor voltage when the throttle is closed while doing 50+ MPH? It should be > 0.1 volts
If both rear O2 sensors are showing the same voltage, I would suspect something else. The odds of BOTH rear o2 sensors failing at the same time is very low.
Check the exhaust for any cracks or leaks where air can enter and change the o2 sensor output votlage.
@@OBD4Everyone it's running rich because I can smell raw fuel in the tail pipe and the short fueltrims goes -20 in bank 1 and bank 2 +10
With no misfire but shake a little bit
Hey 100ega,
I see... yes that would be rich :)
As you probably already know, raw fuel or a VERY rich mixture will kill the catalytic converter very quickly.
If you haven't already, try cleaning the Mass Air Flow (MAF) sensor with a MAF approved cleaner. Sometimes when they get dirty, they under report the airflow at low load and over report at high load. I don't think this will fix anything, but then we can rule it out and assume accurate air flow measurement.
What are the fuel trims and o2 sensor voltages at wide open throttle and when the throttle is closed?
@@OBD4Everyone I allready replaced the MAF sensor,
with the throttle closed fuel trims bank 1 is (-20 ) and bank 2 is (+10) the o2 sensor upstrim is normal .1v -.9v the downstrim is about .8v steady
at wide open throttle everything goes to normal trims around zero
Hey 100ega,
At wide open throttle, does the engine seem to make full power? I'm just wondering about a partly blocked catalytic converter. That might explain why the rear O2 sensors don't change much and the front O2 sensors switch as they should.
If possible, remove the front and rear O2 sensor and measure the exhaust pressure while revving the engine. It should be less than 3-4 PSI. (some exhaust systems have 2 cats per bank and the rear o2 sensor is between them)
Any chance the O2 sensors are plugged into the wrong bank?
(On some cars, it is VERY easy for this to happen)
To quickly check, unplug the O2 sensor on bank 1, and make sure the new trouble codes refer to bank 1.
With the engine running, create a small vacuum leak and see if the front O2 sensors shift to lean. Then try to add propane gas (from a hand held plumber's torch) to the intake and see if they switch to rich. This should help to verify the front O2 sensor operation.
Pull the spark plugs and compare them. Are the plugs on bank 1 much blacker than bank 2? Since bank 1 is rich, they should be. Clean them up or replace them if they are due.
You need to determine why bank 1 is so rich.
Is it fuel (leaking injector), evap system (flooded charcoal canister) or an air restriction (carbon build up on valves, catalytic converter) or a bad O2 sensor.
• The exhaust pressure test will indicate if the cat is blocked. (Yes, I know this test is not easy to do)
• The vacuum leak/propane gas will indicate if the front O2 sensors are functioning properly.
• Disconnecting a pre-cat O2 sensor will verify the sensors aren't plugged into the wrong bank.
If the mileage is high, and you plan on keeping the car a long time, you could replace all 4 O2 sensors and see what happens.
Good luck and let me know what happens next :)
can you add a link for the obd reader?
Hey al gibson,
The next episode to be released is "The Ultimate OBD Scanner Buyers Guide & Speed Test" where 11 different OBD scanners are tested. It is in post production right now and should be out in a few week.
Very good video, thanks for that. But I am a bit confused with my 2009 honda odyssey, it has the code P0430 but it shows voltage 0 in Bank 1 Sensor 1, I already bought a new sensor for Bank 1 Sensor 1, it shows the same 0. Could be the reason of the P0430 a problem with bank 1 sensor 1 cable? What is the job of the O2 sensors bank 1 sensor 1 ?. Thank you again.
Hey Alexander Silva,
I think you are viewing the wrong PID and that is why it is showing zero.
Different manufactures will use different names for the front O2 sensor.
To find the correct sensor name / PID, run the diagnostic report. In the mode $01 section make a note of the exact wording where it mentions bank 1, sensor 1.
Can a bad cable cause a P0430? Yes.
The sensor 1 reports the air/fuel ratio to the engine computer and it uses this feedback to monitor the running condition of the engine. At idle and low engine load it will show a "lambda" of 1.0 which is the same as an air/fuel ratio of about 14.7. Operating the engine with 14.7 pounds of air, for every pound of fuel is the best for emissions.
I have a b2s2 at idle graphed at 0.100 being the highest to 0.060 at lowest up and down on the graph . at idle to 0.080 in the top right corner. ...I get these numbers bc I took a screen shot photo of my obd scan tool graph.... my question is while held at 2k rpm the graph did go up, than just started going down and keep going down until than I let off the gas and it went back to up and down ....?? I'm fighting with. p0421 and not sure if it's an o2 sensor or the warm up cat or the main cat I have a v6 3.0. Eclipse gts I believe 4 sensors and 2 cats . Today first day trying to read o2 sensors as we speak and video watching . Or if anybody knows how to or what to check for when dealing with p0421 ???
Hey jedidias,
How many miles are on the rear O2 sensors?
Look at the Mode $01 long & short fuel trims or run a diagnostic report from OBD Fusion. (Diagnostics > Report)
If you add the SHORT and LONG fuel trims they should be between about -10% to +10% for a properly running engine.
To compare the pre cat and post cat sensors, graph:
• bank 2, sensor 1
• bank 2, sensor 2
-and-
• bank 1, sensor 1
• bank 1, sensor 2
Since your trouble code is from Bank 2, the bank 1 graph should show you what the output voltage should look like.
Testing the O2 Sensors:
Have someone drive the car and:
At full throttle, both sensors should be rich and have more than about 0.8 volts.
After full throttle, let off the gas and both sensors should be lean and have less than about 0.2 volts.
If these voltages are not reached, I would suspect the sensor is due for replacement.
Testing the Catalyst:
Have someone drive the car at a steady speed (30-50 mph) and compare the output of the front (sensor 1) and the rear (sensor 2) O2 sensors:
• The front sensor should be switching between rich (> 0.8 volts) and lean (< .2 volts) fairly often.
• The rear sensor should be switching between rich (> 0.8 volts) and lean (< .2 volts) much slower
if the catalytic converter is working properly.
Good luck with the next steps and please let me know your progress!
@@OBD4Everyone I'm going to look tomorrow and I think 112k miles on the pipes. Thank you for the info I'm new at reading o2s and been watching lots of videos on how to read them . I even got a inferred thermometer gun to check the heat from the cats . Bc p0421 is frustrating . And this car parts aren't cheap . And if it's a a bad cat idk if gut it or replace it . Or o2 sensor to try clean it or buy OEM or autozone .
So 0.5 , 0.6 should be good ?
If you are talking about the voltage at the rear O2 sensors during low engine load, they should switch between 0.1v and 0.9 volts.
If the measured voltage is always 0.5 - 0.6 that might indicate a problem.
@@OBD4Everyone at idle it's. 05 or 6 then it fluctuates at higher rpm but the other o2 sensor reads like 1.9 at idle
If the other sensor is at 1.9 volts, then your rear o2 sensors may not be be O2 sensors, but air/fuel sensors.
Can you make a video on how to setup Realtime charts for torque pro ?
i'm struggling to make it run.....
Hey tonytherunner,
Sure, I will add that to the production schedule. It will be a few months before it is released, as two other episodes are in post production right now.
Thanks
I have a question about PID list I tried torque scan for more pid and no result,where i can find a list of PID to upload o torque.is a Land rover freelander 2001 petrol ?
Hey tonytherunner,
Only the generic OBD PIDs that the ECU says it supports, will be shown in Torque. This is shown in green when doing a Torque Scan.
The generic supported PIDs are also shown in "Adapter Settings" under "Available Sensors".
I suspect you are looking for the "enhanced diagnostic" PIDs where you can find out much more information. However, this information is VERY hard to come by and to figure out. My only suggestion is an online search to find them, if they exist.
All I found was this:
landroverforums.com/forum/discovery-ii-18/great-%2Acheap%2A-diagnostic-tool-investment-43948/
You might want to join a Land Rover forum or facebook group, sometimes you can find a lot of good information.
Good luck on your search!
well i ask on land rover forum and torque forum but seem like no one know .
i was thinking i find pid like FORD GM NISSAN ect ,even if i need to buy.
on playstore i see something name advanced LT EX for infiniti ect but nothing for me.
i searched for oil temp sensor because torque don't read , i need a mathematical formula so i not that easy,i tried some honda forum formula but doesn't work .
anyway thanks for help