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Q: Check engine light on and the P0128 code is showing

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My car has the check engine light on. There is a P0128 code, which shows that the thermostat doesn't work correctly. The PCM shows that the engine isn't at the right temperature that it should reach from the startup. The gauge goes from cold to normal temperature in a few minutes from the startup. Is this an issue with the thermostat or something else?

A: The P0128 code is most likely caused by one...

The P0128 code is most likely caused by one of the following three issues: a faulty thermostat, a faulty engine coolant temperature sensor, or a processing error in the ECM. The coolant temperature needs to measured using a temperature gun from a cold start and compared to what the ECM is seeing all while the temperature sensor voltage drop is being monitored. This will locate the cause of the issue. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the necessary equipment and training to conduct a thorough diagnosis for the cause of the code, and then have the correct repairs made.

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