Coolant Temperature Sensor/Switch (For Computer): Description and Operation

The Engine Coolant Temperature (ECT) sensor (3) is a thermistor, a resistor that changes value based on temperature. The ECT is mounted in the engine coolant stream. The ECT sensor's resistance varies with the coolant temperature. Low coolant temperature produces high resistance, approximately 100,000 ohms at -40°C (-40°F), while high temperature causes low resistance, approximately 70 ohms at 130°C (266°F).

The PCM supplies a 5 volt signal to the ECT sensor through a resistor in the PCM and monitors the terminal voltage. This forms a series circuit to ground through the ECT sensor, causing high sensor resistance (low temperature) to result in high PCM terminal voltage. When the resistance of the ECT sensor is low (high temperature), the terminal voltage is reduced. The terminal voltage indicates engine coolant temperature to the PCM.

A hard fault in the ECT sensor circuit should set either a DTC P0117 or P0118. These DTCs indicate a malfunction in the engine coolant temperature circuit. Most such malfunctions can be corrected by either repairing a wiring problem or replacing the sensor. Consult the appropriate DTC table to properly repair any problems with the ECT circuit.