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 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 or DTC P0115; an intermittent fault should set a DTC P1114 or P1115. The ECT sensor also contains another circuit which is used to operate the engine coolant temperature gauge located in the instrument panel.