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.
When is the code detected?
The engine coolant temperature does not reach to specified temperature even though the engine has run long enough.
Engine Light ON (or Service Engine Soon Warning Light)
Indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.