Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40° F [-40° C]) while high temperature causes low resistance (70 ohms at 266° F [130° C]).

The powertrain control module (PCM)/engine control module (ECM) supplies 5 volts to ECT sensor through a resistor in PCM/ECM and measures change in voltage. The voltage will be high when engine is cold, and low when engine is hot. By measuring change in voltage, PCM/ECM can determine coolant temperature. The engine coolant temperature affects most of systems that PCM/ECM controls. A failure in ECT sensor circuit should set a diagnostic trouble code P0117 or P0118. Remember, these diagnostic trouble codes indicate a failure in ECT sensor circuit, so proper use of chart will lead either to repairing a wiring problem or to replacing sensor to repair a problem properly.