1.Disconnect the connector from the coolant temperature sensor (temperature sending unit). Measure the resistance across the terminal pins using a digital multimeter and write this value down. Do this before running the engine, it must be cold for this test.
2.Reconnect the connector to the sensor. Start the engine and allow to warm up for two minutes.
3.Disconnect the connector from the sensor again. Measure the resistance across the terminal pins again.
4.Determine the difference in the two readings. If there is not at least 200 ohms difference in the two readings the sensor is dirty or defective.
5.Remove the sensor from the engine if the ohms reading is too low. Clean the sensor, removing any deposits, and repeat the test. Another low reading indicates the sensor is defective and must be replaced.