The Engine Coolant Temperature (ECT) sensor is a thermistor, a resistor which varies the value of its resistance in accordance with temperature changes. The change in the resistance values will directly affect the voltage signal from the sensor to the ECM/PCM. As the sensor temperature increases, the resistance values will decrease and vice versa. Performing the following test will set a diagnostic trouble code and illuminate the Service Engine Soon light. Clear the diagnostic trouble code after performing the tests and making the necessary repairs. The engine coolant temperature sensor threads into a coolant passage at the front of the engine near the thermostat housing. Check the terminals in the connector and the wires leading to the sensor for looseness and breaks. Repair as required. Before checking the engine coolant temperature sensor, check the voltage supply and ground circuits from the ECM/PCM. Disconnect the electrical connector from the engine coolant temperature sensor and connect a voltmeter to the two terminals of the harness connector. Turn the ignition key On-the voltage should read approximately 5.0 volts. If the voltage is incorrect, check the wiring from the engine coolant temperature sensor to the ECM/PCM. If the circuits are good, have the ECM/PCM checked at a dealer service department or other properly equipped repair facility. With the ignition switch OFF, disconnect the electrical connector from the engine coolant temperature sensor. Using an ohmmeter, measure the resistance between the two terminals on the sensor while it is completely cold. Reconnect the electrical connector to the sensor, start the engine and warm it up until it reaches operating temperature, disconnect the connector and check the resistance again. Compare your measurements to the resistance chart. If the sensor resistance test results are incorrect, replace the engine coolant temperature sensor. A more accurate check may be performed by removing the sensor and suspending the tip of the sensor in a container of water. Heat the water on the stove while you monitor the resistance of the sensor. Wait until the engine is completely cool before beginning this procedure. Drain the cooling system. Disconnect the electrical connector from the sensor and carefully unscrew the sensor. Before installing the new sensor, wrap the threads with Teflon sealing tape to prevent leakage and thread corrosion. Installation is the reverse of removal.
