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P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Severity: Low Usually safe to keep driving, but get it checked.
What does P0128 mean?
The engine is not reaching its normal operating temperature within the expected time. The most common cause is a thermostat stuck partially open.
Generic OBD-II definition: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). Confirm applicability in your vehicle service information.
Common causes
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
- Cooling fan running all the time
Symptoms
- Temperature gauge reads low
- Weak heater output
- Lower fuel economy
Can I keep driving with P0128?
Severity: Low Usually safe to keep driving, but get it checked.
If the check engine light is flashing or the car runs badly, stop driving and have it checked.
Typical fixes
- Check coolant level
- Replace thermostat
- Test ECT sensor and fan operation