P0128
Coolant Thermostat (Coolant Temperature Below Regulating Temperature)
Your car's engine isn't warming up to its normal operating temperature quickly enough, or it's running too cool, usually because the thermostat is stuck open.
The powertrain control module (PCM) has detected that the engine coolant temperature is not reaching the proper operating temperature within a specified time frame or is not maintaining it.
Common causes
- high
Faulty Thermostat (stuck open)
This is the most common cause; a thermostat stuck in the open position allows coolant to constantly flow through the radiator, preventing the engine from reaching optimal temperature.
Typical part cost: $50-150
- medium
Faulty Engine Coolant Temperature (ECT) Sensor
A malfunctioning ECT sensor can send incorrect temperature readings to the PCM, making it think the engine is too cold even if it's at normal operating temperature.
Typical part cost: $30-100
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First things to check
- 1Verify engine coolant level is full and free of air bubbles.
- 2Monitor engine coolant temperature with a scan tool from a cold start to observe warm-up rate and final temperature.
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Related codes
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Frequently asked questions
What does a thermostat do?
The thermostat is a valve that regulates the flow of coolant to the radiator. It stays closed when the engine is cold to help it warm up quickly, then opens to allow coolant to flow to the radiator and prevent overheating.
Will a P0128 code cause my car to overheat?
No, a P0128 code indicates the engine is running too *cold*. An engine running too hot would typically trigger codes like P0217 (engine over temperature) or P1299 (cylinder head over temperature).
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