Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a thermistor (temperature sensitive resistor) located in an engine coolant passage. The Powertrain Control Module (PCM) supplies and monitors a 5-volt signal to ECT sensor through a resistor in PCM. This monitored 5-volt signal is then reduced by resistance of the engine coolant temperature. When coolant temperatures are low, ECT sensor resistance is high, and a high monitored voltage signal is seen by the PCM. When coolant temperatures are high, ECT sensor resistance is low, and a low monitored voltage is seen by the PCM. After engine start-up, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens.

Engine coolant temperature signal is used in the control of most systems the PCM controls (i.e., fuel delivery, ignition timing, idle speed, emission control devices) and as an enabling criteria for some diagnostics. After a vehicle has been parked overnight, ECT and IAT sensor signals (resistance and temperature) should be close to same reading. An ECT sensor which is out of calibration will not set a Diagnostic Trouble Code (DTC) but will cause fuel delivery and driveability problems. Failure in ECT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related DTC.