The Engine Coolant Temperature (ECT) sensor tells the ECM how hot the engine is. The ECM uses this information to control fuel mixture, ignition timing, cooling fan operation, and many other systems. When it fails, you get hard cold starts, rough running, poor fuel economy, fans running constantly or not at all, and check engine codes. Replacement is one of the easiest sensor jobs.
Symptoms of a Failed ECT Sensor
Hard starting when cold (the ECM does not know the engine is cold and does not enrich the mixture appropriately), rough idle when warming up, black smoke from the exhaust (over-rich mixture from the ECM thinking the engine is colder than it is), poor fuel economy, the cooling fan running constantly or not running at all, the temperature gauge reading wildly inaccurate or fixed at one position, and codes P0115 through P0119 indicating ECT circuit faults.
Two Different Sensors
Many vehicles have two coolant temperature sensors: one for the ECM (engine management) and a separate one for the dashboard temperature gauge. Make sure you replace the correct one. Your workshop manual identifies their locations. The ECM sensor is usually located near the thermostat housing or on the cylinder head where it can sense coolant entering or leaving the engine. The gauge sender is often in a different location.
Testing Before Replacement
Before replacing, confirm the sensor is faulty. Disconnect the sensor’s electrical connector and measure resistance across the sensor’s terminals with a multimeter. Compare to the workshop manual’s resistance-vs-temperature chart. Cold (around 20 degrees Celsius), expect roughly 2,000 to 3,000 ohms. Hot (around 90 degrees Celsius), expect 200 to 300 ohms. Out-of-range values or no continuity at all confirm sensor failure. See our engine sensor specifications guide.
Replacement Procedure
The engine should be cold for safety. Drain a small amount of coolant from the radiator (just enough to drop the level below the sensor) or be ready to plug the hole quickly. Disconnect the electrical connector. Use a deep socket or open-ended spanner to unscrew the sensor – they are usually 19 mm or similar. Coolant may dribble out as you remove the sensor. Quickly fit the new sensor (some have a copper or aluminium crush washer that should be replaced) and tighten to specification. Reconnect the connector, top up the coolant, run the engine, and watch for leaks.
Sensor location, torque specifications, and resistance values are vehicle-specific. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
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