What is the correct way to check a coolant temperature sensor?

To check a coolant temperature sensor, first, ensure the engine is off and cool. Disconnect the sensor's electrical connector, and use a multimeter to measure the resistance across the sensor terminals; compare this reading to the manufacturer's specifications for temperature vs. resistance. Additionally, you can test the sensor at different temperatures by submerging it in water at various temperatures and checking if the resistance changes accordingly. If the readings are out of spec, the sensor may need to be replaced.