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Engine Coolant Temperature Sensor (ECT): Understanding The Structure, Symptoms, Maintenance, And Costs

By R&D
Published on December 22nd, 2023

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The Engine Coolant Temperature (ECT) sensor is a small yet critical component in a vehicle's cooling system. Its function is to monitor the engine's operating temperature. The vehicle’s Engine Control Module (ECM) uses this information, along with inputs from the oxygen sensor(s), to determine when the engine is warm enough to enter closed-loop feedback fuel control.

Overview of the Engine Coolant Temperature Sensor

The ECT sensor is typically threaded into the cylinder head, where the sensor tip can contact the engine's water jacket. The ECT sensor provides essential engine operating temperature data to the ECM. The ECM also uses the signal from the ECT to perform several other functions based on current engine temperature.

When the engine is cold, the ECT sensor detects the low coolant temperature and supplies this information to the ECM. In response, the ECM enriches the air-fuel mixture during startup to help the engine run smoothly.

  • Spark advance and retard - To minimize vehicle emissions, spark advance is limited or inhibited until the engine reaches normal operating temperature.
  • Exhaust Gas Recirculation (EGR) Valve Control - EGR flow is blocked to improve "cold engine" drivability.
  • Canister Purge - Canister purge is inhibited until the engine reaches normal operating temperature to improve "cold engine" drivability.
  • Idle Speed Control - Operation of the throttle kicker or idle speed control to improve "cold engine" performance.
  • Torque Converter Clutch Lock-up - Activation of torque converter clutch lock-up for optimized operation, particularly during cold engine phases.
  • Start-up Fuel Enrichment (fuel injected engines) - Fuel injector "dwell" time is increased to "enrich" the fuel mixture and improve "cold engine" performance.

As the engine warms up, the ECT continues to monitor the coolant temperature, enabling the ECM to adjust the fuel injection and ignition timing for optimal performance and fuel efficiency.

  • Fan Control - Operation of the electric cooling fan when a preset temperature is reached.

Common Symptoms of a Faulty ECT Sensor

A malfunctioning ECT sensor can result in various issues with your vehicle's engine and overall performance. Common symptoms can include:

1. Engine Overheating: A faulty ECT sensor may not accurately sense the engine's temperature. This can result in improper cooling system regulation and potential overheating.

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Engine Overheating

2. Constant High Engine Temperature: If the ECT sensor sends incorrect high-temperature readings to the ECM, the engine may run too hot, causing a decrease in performance and fuel efficiency.

3. Rough Idling: A malfunctioning ECT sensor can affect the engine's idle speed, causing it to fluctuate or remain unstable.

4. Poor Fuel Efficiency: Since the ECT sensor signal is used in determining the ideal air-fuel mixture, a faulty sensor can result in reduced fuel efficiency.

5. Malfunction Indicator Lamp (MIL): If the ECT sensor is sending irregular or out-of-range data, it can cause the vehicle's MIL to illuminate.

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The MIL light on

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Testing Tips:

The operation of resistance-type coolant temperature sensors is easily checked using an ohmmeter in accordance the following procedure. BE SURE the sensor is cold before testing.

1. If necessary, drain enough coolant from the engine to prevent spillage when the sensor is removed.

2. Unplug the wiring harness from the sensor and remove the sensor from the engine.

3. Connect an ohmmeter to the sensor’s signal pins. Note the sensor’s resistance.

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Connect the Ohmmeter

4. Heat the sensor tip with a blow dryer or other low-temperature heat source. DO NOT use a propane torch or other high-temperature heat source. DO NOT overheat the sensor.

5. Observe the ohmmeter and verify that the sensor’s resistance decreases as heat is applied.

6. Remove the heat source and disconnect the ohmmeter from the sensor. Allow the sensor to cool.

7. Apply thread sealant to the sensor threads to prevent coolant leaks, then reinstall the sensor in the engine. DO NOT overtighten the sensor when installing.

8. Reconnect the wiring harness to the sensor’s connector.

Cost of ECT sensor replacement

The cost of replacing an ECT sensor can vary depending on the vehicle make and model, as well as the labor costs in your area. In general, ECT sensors are relatively affordable, with prices ranging from $20 to $100.


The ECT sensor is a vital component in your vehicle's cooling system, influencing engine performance and fuel efficiency. Understanding how the ECT sensor works and being able to diagnose potential issues can save you time and money on vehicle maintenance. Regular inspection and maintenance of the ECT sensor will help keep your vehicle running smoothly and efficiently.

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