How to tell whether the thermostat is to blame
Weak heater and higher fuel use? One graph answers the question in ten minutes.
Start the car cold and open the temperature reading
This check only works from cold — overnight or after a few hours parked. Open the live data and leave the coolant temperature on screen.
Time how long it takes to reach operating temperature
Operating temperature is about 85–105 °C. Idling, a warm-up takes 10–15 minutes; driving, less. If after twenty minutes of driving it sits at 60–70 °C, the thermostat is stuck open: coolant runs the long loop from the start.
Watch what the temperature does next
On a healthy car the temperature is almost flat once warm, wobbling slightly when the fan cuts in. A saw-tooth of tens of degrees means a sticking thermostat. A steady climb above 105 °C is no longer the thermostat but overheating: stop the car.
Tie it to what you noticed while driving
A thermostat stuck open gives a weak heater in winter and higher fuel use: the unit keeps the mixture rich while it thinks the engine is cold. Code P0128 — «engine does not reach operating temperature» — is about exactly this case.
Make sure the sensor itself is not the problem
If a clearly hot engine still reads near zero or pinned at a limit, the sensor is lying rather than the thermostat. Codes P0116, P0117 and P0118 point the same way. Replace the thermostat when the sensor tells the truth and the temperature is still wrong.