Coolant Temperature Sensor Test

Most coolant temperature sensors resemble a large nut with an electrical connector on top.
Most coolant temperature sensors resemble a large nut with an electrical connector on top. | Source

The engine coolant temperature (ECT) sensor test is simple and can help you fix your car faster. You can do it at home using a digital multimeter and a cooking thermometer.

A bad engine coolant temperature sensor affects engine performance:

* It may cause a constant lean air/fuel mixture, causing the engine to stall or idle rough.
* Or it may cause a constant rich air/fuel mixture, causing an increase in emissions and fuel consumption.
* On modern vehicles, a bad ECT sensor will upset ignition timing.
* On some vehicle models, a bad ECT sensor may upset the transmission, cooling fan, and temperature gauge as well.

But before you blame the coolant temperature sensor for your engine problems, though, use this guide to test the sensor to confirm that you actually need to replace it. The test only takes a few minutes.

Tools and Items You'll Need:
Digital multimeter
Kitchen thermometer
Wrench - ratchet and socket set, if you need to remove components
Paper and pencil

Locate the Coolant Temperature Sensor

Open the hood of your vehicle to find the coolant sensor.

1. Depending on your particular vehicle model, you may find the sensor mounted on the cylinder head or intake manifold. However, one of the most common locations is on or near the thermostat housing. You can find the thermostat housing by following the upper radiator hose, which connects to the thermostat housing on the engine side.

2. Look for a large nut with an electrical connector on top, and two electrical wires coming from the connector — on most modern vehicles — or one wire — on older vehicle models.

3. Some sensors are buried under the intake plenum, specially on large trucks, and you need to remove the intake just to gain access to it.

If you still have trouble locating the ECT sensor, consult your vehicle service manual. You can buy a service manual for your particular car make and model in most auto parts stores or online. Check the Amazon ad below.

Use a digital multimeter to test your coolant temperature sensor.
Use a digital multimeter to test your coolant temperature sensor. | Source

Coolant Temperature Sensor Test

Now that you've located the ECT sensor on your vehicle, you're ready to troubleshoot it.

1. Unplug the sensor electrical connector.

2. Get the engine surface temperature using an infrared thermometer or suitable cooking thermometer. Take the engine temperature on a location near the coolant temperature sensor.

Before going on to the next step, let's divert here for a second:

Okay, at this point you may be wondering why you need to take the engine temperature to troubleshoot the sensor. The main reason is that you are trying to check two common, potential failures here, the ECT sensor and the thermostat.

Let's say that the thermostat on your vehicle got stuck in the open position. This will not allow the engine to reach operating temperature because the coolant is flowing continuously. If you were to test the coolant temperature sensor alone, you may think that it failed because its resistance value has remained at about 1500 or 2100 ohms, for example, when in fact the sensor is reporting the coolant actual temperature and it's working properly.

You could relay on the temperature gauge on your dashboard. However, on some vehicle models this gauge works through the ECT sensor as well. So, if the sensor doesn't work properly, your temperature gauge won't be of much help either.

By using the thermometer, it won't take you long to figure out that the thermostat isn't working. You'll notice the engine's temperature is not rising above 85 or 90 degrees, for example.

On the other hand, if the thermostat works fine, the engine temperature will reach about 200F (93C) and will drop afterwards as the thermostat opens. So you eliminate the thermostat as another possible failure.

Okay, now let's go on to the next step.

3. Take a note of the temperature reading.

4. Now, using your ohmmeter, measure the resistance value of the coolant temperature sensor by hooking up one of the meter's leads to one of the terminals on the sensor electrical connector, and the other lead to the other terminal on the sensor electrical connector.

On vehicles with old, single-wire sensors, hook up the meter leads to the terminal on the connector and the sensor's body (ground) to take your reading.

5. Check your vehicle service manual for the correct resistance value for your ECT sensor. However, not all service manuals have this information.

Most sensors of this type have a resistance value of 3000 ohms or more at about 55F (13C). You may want to try searching online for a resistance value table for your particular ECT sensor, if you know your sensor's brand.

However, whether you find the resistance values for your particular ECT sensor, continue with this tests anyway, the sensor's behavior and the temperature readings may give you a clue to its operating condition.

6. Make a note of the sensor's resistance.

Now you're going to take another pair of readings.

7. This time, start the engine and let it idle.

8. Set the transmission to Neutral and apply the parking brakes.

9. Wait for about one to two minutes and measure the engine temperature and the sensor's resistance as you did before.

10. Make a note of these new pair of values.

11. Without turning off the engine, wait for about one to two minutes and repeat this process again.

12. Take another pair of readings in about one to two minutes again, always noting the value readings.

13. Then turn off the engine.

On the next video, you'll see an alternative method to test a sensor using water. Of course you need to remove the sensor from the vehicle to use this method. However, the video will give you a visual reference about how to test your ECT sensor. Check it out.

Checking Your Readings

Now, compare your figures to the resistance and temperature ratings for your particular sensor listed in your vehicle service manual.

If you don't have the reference resistance values for your ECT sensor, make sure that your sensor resistance readings decreased proportionally as the engine temperature increased. This will indicate that your coolant sensor is responding to engine temperature.

Also, make sure the engine reached operating temperature at about 200F (93C) before dropping. If engine temperature didn't change much throughout your readings, the thermostat is stuck open and you need to replace it.

When should you replace the sensor?

* Whether you got an unusual reading or not, check that the wiring and electrical connector is free of corrosion. If one or more wires show sign of damage, repair them. Remove corrosion from the sensor's electrical terminal using electrical contact cleaner and repeat your test, if necessary.

* If the ECT sensor's resistance didn't change as the engine temperature rose, replace the sensor.

* If your sensor only registers infinite resistance, the sensor has a broken internal contact and you need to replace it.

* If the sensor only registers zero resistance, the internal contacts are shorted. Replace the sensor.

The thermostat housing is one of the most common locations for a coolant temperature sensor.
The thermostat housing is one of the most common locations for a coolant temperature sensor. | Source

The coolant temperature sensor test is rather simple. It only takes a few minutes and you don't even have to remove the sensor from the engine for the troubleshooting procedure. Following the steps outlined in this guide will help you quickly determine whether you need to replace the sensor and will also check the thermostat operation at the same time by using a thermometer. Also, make sure to inspect the sensor electrical terminal for damage and corrosion, and check the circuit wires for possible damage. It won't help that your ECT sensor tests Okay when it has to operate with a damaged terminal or wire.

Test Your Knowledge of Failing ECT Sensors

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