THERMOSTAT broken symptoms - how to recognize a thermostat fault in the car?

If the engine temperature gauge is not in the 90 degrees Celsius position while driving, it means that the thermostat is faulty, i.e. blocked in the open position, i.e. non-stop, the cooling fluid is directed to the radiator. And it should be closed to a temperature of 90 degrees, above it should open. Up to 90 degrees fluid circulates only in the engine and heater (Heat exchanger inside the cabin - responsible for heating in the car).
The thermostat can be damaged every few years, this crap is now produced and we can not help it. I have already mentioned a few and every time after the exchange suddenly everything is perfect.
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