The engine coolant temperature (ECT) 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 of about 100,000 Ohms at -40 °C (-40 °F). High temperature causes a low resistance of about 70 Ohms at 130 °C (266 °F).

The PCM supplies a 5-volt signal to the ECT sensor through resistors internal to the PCM and then measures the voltage after the internal resistor. This signal 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 of the systems that the PCM controls.

The Tech 2 displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 85 °C (185 °F). It then stabilizes when the 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 will set DTC P0117 or DTC P0118. An intermittent fault will set a DTC P1114 or P1115.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Engine Temperature Sensor: Description and Operation

Use the Manual With the Right Hardware

When to See a Mechanic

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