DTC P0128: Coolant Thermostat Stuck Open: Condition
WARNING: This page is about a different car, the 2002 Mazda Protege5 and 2002 Mazda Protege. However, it is still accessible from the selected car via links, so may be relevant.
DTC is set when:
- Vehicle speed is more than 15 MPH A/T (25 MPH with M/T)
- Intake air temperature is more than 14°F (-10°C)
- Difference between engine coolant temperature and minimum intake air temperature readings is less than 6°C when engine is first started
- Engine coolant temperature is less than 95°F (35°C) when engine is first started.
- PCM determines difference between actual and expected engine coolant temperature is too great.
In setting DTC, PCM has determined that coolant thermostat is stuck open. This is an intermittent monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:
- Engine Coolant Temperature (ECT) sensor malfunction.
- Coolant thermostat malfunction.
- PCM malfunction.