Engine Coolant Temperature Sensor

What Is An Engine Coolant Temperature Sensor?

The engine coolant temperature sensor, or engine coolant temperature switch, is a sensor that measures how hot the engine cooling system is getting. Its acronym is ECT. Some vehicles have two separate sensors, one dedicated for the temperature gauge on the gauge cluster and the other for the Engine Control Module (ECM) to use as a reference for coolant temperature. The ECM uses this information in order to calculate how much fuel to add to the engine. When the engine is cold, it adds more fuel to help warm up the catalytic converter. Once the engine is up to operating temperature, it will decrease the amount of fuel it is injecting for optimum performance, fuel economy, and lower emissions. Most of these ECT sensors are two or one wire sensors, however, some vehicles are equipped with a four-wire sensor.

The ECT sensor can fail in two different ways

  1. It gets stuck reading full cold; when this happens, the ECM will inject a lot of fuel which can cause an extremely rich running engine that can emit black smoke from the tailpipe, the exhaust may smell like fuel, poor fuel economy, the check engine light may illuminate, the engine may overheat due to the cooling fans not operating correctly, and if driven for an extended period of time in this condition can cause failure of the catalytic converter.
  2. If it fails on the hot side of things, it can cause the cooling fans to stay on, reduce engine power, a/c not to work, the check engine light may illuminate, and the temperature gauge in the vehicle can register hot at all times.

How To Test An Engine Coolant Temperature Sensor?

Tools needed:

  • Vehicle Specific Service Manual
  • Basic Hand Tools
  • Multimeter
  • Thermometer
  • Hot Water
  1. The first step is to locate and remove the ECT sensor. (make sure to do this when the engine is cool to the touch)
  2. Next, check its resistance value using your multimeter on the ohms setting. Compare the value you read to your vehicle’s service manual at the temperature you tested the sensor at.
  3. Now put it in hot water while measuring the resistance. It should change as the temperature changes. If it doesn’t meet the specification per the service manual, replace the sensor with a new one. (Make sure to fill the coolant after you’re done replacing it) 

Check out this video for a visual of how to do this.

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