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Q: P0128 code, but coolant is full

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I got this code when I checked the computer on my vehicle: "P0128". I looked up the code and it means "Coolant Temperature Below Thermostat Regulating Temperature". This code showed up about a week ago. I cleared it, and the vehicle actually passed inspection right after.

Later the same day, the Check Engine Light came on again, with the same P0128 code. My coolant level is just fine, so I apparently don't have any leaks. What might be causing the code? Thermostat perhaps?

This is most likely caused by the thermostat partly stuck open. The coolant sensor would need to be tested to make sure it is not bad, though. I would test the thermostat at the housing with a temperature tester, and compare it to the coolant sensor readings.

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