Let the engine cool completely
Locate the coolant temperature sensor
Disconnect the sensor electrical connector
Inspect the connector and wiring for damage, corrosion, or loose pins
Check the sensor resistance with a multimeter
Compare the resistance reading to the manufacturer’s temperature-resistance chart
Test the sensor in cold coolant or ice water
Test the sensor in warm or hot water
Verify the resistance changes smoothly as temperature changes
Check the sensor supply voltage at the connector with key on
Check the sensor ground or reference signal
Scan live data with an OBD2 scanner
Compare the displayed coolant temperature to actual engine temperature
Wiggle the harness while monitoring readings for intermittent faults
Replace the sensor if readings are out of specification
Clear any fault codes after repair
Recheck operation after the engine reaches operating temperature