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

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream.




Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while high temperature causes low resistance (70 ohms at 130°C/266° F).




The PCM supplies a 5.0 volt signal to the engine coolant temperature sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the PCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.




The scan tool displays engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other.

A hard fault in the engine coolant sensor circuit should set DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage, or DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage, an intermittent fault should set a DTC P1114 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage, or DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage. The DTC Diagnostic Aids also contains a chart to test for sensor resistance values relative to temperature.




The ECT sensor (3) also contains another circuit which is used to operate the engine coolant temperature gauge located in the instrument panel.