Low engine coolant level
Insufficient warm uptime
Leaking or stuck open thermostat
Faulty engine coolant temperature sensor
Engine coolant temperature sensor harness is open or shorted
Engine coolant temperature sensor circuit poor electrical connection
Engine Light ON (or Service Engine Soon Warning Light)
The Engine Coolant Thermistor (ECT) is a 2-wire thermistor. As Engine coolant temperature increases, thermistor resistance & signal voltage decreases. The Engine Control Module (ECM) monitors ECT voltage to help determine injector pulse width, ignition timing, and idle control. The Transmission Control Module (TCM)/ECM uses ECT voltage for shift timing & quality. The Diagnostic Trouble Code (DTC) indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.