P0128 - Coolant Thermostat Malfunction
- Leaking or stuck open thermostat
- Low engine coolant level
- Insufficient warm up time
- Faulty engine coolant temperature sensor
- Engine coolant temperature sensor harness is open or shorted
- Engine coolant temperature sensor circuit poor electrical connection
If cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problem.