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 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. The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 90°O (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.

Engine coolant temperature affects most systems the PCM controls. A hard fault in the engine coolant sensor circuit should set DTC P0117 or DTC P0118; an intermittent fault should set a DTC P1114 or P1115. The DTC Diagnostic Aids also contains a chart to check for sensor resistance values relative to temperature.