How to Test a Coolant Temperature Sensor

A warning light for an overheating engine.
  • 2-5 hours
  • Intermediate
  • 25-50
What You'll Need
Drop light
Digital volt-ohm meter
Replacement sensor

The coolant temperature sensor is what tells the car's computer what the engine's running temperature is. This will then translate to how the engine runs. Before the automated systems in vehicles, this was the sole purpose of the temperature sensor. Now, with this special sensor, the engine will adjust itself so that it runs smoothly even on the coldest days and does not overheat once the temperature rises. Testing this coolant temperature sensor, and then replacing it if needed, is a very easy process.

Step 1 - Locate Coolant Temperature Sensor

The coolant temperature sensor is located on the engine block under the hood. Pull the latch for the hood and open it, making sure it is secure before letting go.

You will search for the sensor within the engine block itself, using a drop light if you need help to see it better. Look at the front of the engine block in the middle of the pulleys. You will see a small terminal sticking out of the block with a wire lead coming from it. This is your coolant temperature sensor.

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Step 2 - Connect Digital Volt-Ohm Meter

Testing the coolant temperature sensor is a very quick process. With the use of a digital volt-ohm meter you can easily tell if the sensor is faulty or not. Connect the black lead of the digital ohm meter to a solid grounding. This can be any solid piece of metal. Next, remove the wire from sensor terminal and connect the red lead to the terminal end of the coolant temperature sensor. Before turning the meter on, set the digital reading to the 20K range.

Step 3 - Check Readings

For the testing process you will need to turn the engine on. Let the engine run for a full two minutes to allow the engine to get up to running temperature. While the engine is running you need to be continually checking the digital ohm meter. You are looking for readings that are more than 200 ohms in variance between a cold and warm engine. If you do not see anything that is more than 200 ohms in difference, the coolant temperature sensor is defective and will need to be replaced.

Step 4 - Check With Cold Sensor

If you have a new sensor on hand and want to check it, you can do this easy test. Connect the black lead of the meter to the body of the cold sensor and the red to the terminal. You should have a reading of approximately 2000 ohms. Check the warm sensor in your engine. You should see a much lower reading on the ohm meter. If not, then it is not working correctly.

Step 5 - Replace Defective Coolant Temperature Sensor

Replacing the coolant temperature sensor is an easy process that anyone can do at home with some basic mechanic's tools. Be sure the cooling system is not pressurized and cooled completely. Remove the lead coming from the terminal and loosen the sensor with a socket or wrench. Replace with a new one and get back on the road in no time.

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