P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Coolant temperature stays below the thermostat opening temperature for too long.
Common causes
- Thermostat stuck open
- Faulty ECT sensor
- Cooling fan running constantly
Symptoms
- Check engine light
- Temperature gauge reads low
- Weak cabin heat
- Higher fuel consumption
What to check
- Replace a thermostat that is stuck open
- Check fan control
- Verify ECT sensor reading
Manufacturers may add their own detail to generic codes. If your vehicle shows a different description for P0128, follow the service information for that make and model.
Related codes
- P0120 Throttle/Pedal Position Sensor/Switch "A" Circuit
- P0121 Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance
- P0122 Throttle/Pedal Position Sensor/Switch "A" Circuit Low
- P0123 Throttle/Pedal Position Sensor/Switch "A" Circuit High
- P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control