OBD
P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
What it means
P0128 means Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).
Check these first
- A thermostat stuck open
- A low coolant level
- A failed engine coolant temperature sensor
- A blocked radiator
- A failing water pump
What to do next
- Check the coolant level
- Monitor the ECT reading against the warm-up target
- Replace the thermostat if it is stuck open
- Test the ECT sensor against actual temperature
- Check the water pump operation
- Clear the code and retest
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Updated
Generic SAE J2012 definition. Last updated 2026-10-10.