Why does my VW (or Audi) never get to full operating temperature?

For a while now my 2013 VW Passat Alltrack 2.0 TDI has rarely reached full operating temperature. I even replaced the main thermostat which is NOT a quick and easy job. Finally, after doing a lot of research I found that this car MAY have another in-line (in hose) thermostat, which could exhibit the same symptoms. I need to service the car so now is a good time to look at replacing the 2nd thermostat.
The small in-line thermostat surely fixed the problem!
The part number of the part is 1K0121113A and it is readily available.
If you are having similar problems then I would suggest replacing (or at least testing, I show you how in this video) this part before replacing the main thermostat. This is a simple 30 minute job from start to end. But you will need to drain the coolant to avoid spillage/waste.