DTC P0128: Coolant Thermostat Stuck Open: Condition
WARNING: This page is about a different car, the 2002 Mazda 626. 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, intake air temperature is more than 14°F (-10°C), difference between Engine Coolant Temperature (ECT) sensor readings and minimum intake air temperature is less than 6°C when engine is first started, engine coolant temperature was less than 95°F (35°C) when engine was first started, or Powertrain Control Module (PCM) determines difference between actual and expected engine coolant temperature is too great. In setting DTC, Powertrain Control Module (PCM) is concluding 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.