What is a Coolant Temperature Sensor? Symptoms & Fixes

Find out if you’re overpaying on car insurance using our cost calculator! Save money by comparing quotes from over 30 of Canada’s top insurance providers!

The best part? It’s FREE!

coolant temperature sensors

A faulty sensor is worse than faulty hardware. With hardware, you can be sure about what is wrong. You just need to have basic knowledge about automobiles to diagnose the defect.

But with sensors, it is usually trial and error. Almost all sensors give similar kinds of warning signals before failing. So, it can get confusing at times.

The engine coolant temperature sensor of my car went bad once. The signs I was getting before it failed were also the most common symptoms of a bad MAP (manifold absolute pressure) or fuel pressure sensor.

Let us see the signs of bad coolant temp sensors and how to replace them.

What is a Coolant Temperature Sensor?

coolant sensor

A coolant temperature sensor is used to measure the coolant temperature inside the engine. It is also known as a CTS or ECT sensor. Its reading also displays the engine temperatures on the dashboard gauge.

By measuring the temperature of the coolant, the ECU also knows how much heat the engine is giving. This is how it determines the engine temperature.

The temperature of the coolant needs to be maintained when the engine is running. If it goes too hot the engine components are at risk of failure. If it goes cold, the vehicle won’t work efficiently.

There is a need to check the coolant’s temperature so that corrective measures can be taken in case of an anomaly.

How Does a Coolant Temperature Sensor Work?

An ECT sensor measures the temperature of the coolant inside the engine. It does so with the help of the ECU. The ECU sends a voltage signal to the resistance offering part of the ECT sensor.

The resistance offered to the current signal sent by the ECU is calibrated such as it determines the accurate temperature of the coolant.

This reading is returned to the ECU which adjusts the fuel mixing ratio, fuel injection timing, ignition timing, and spark timing according to it. This is how a coolant temperature sensor works in harmony with the ECU to aid the proper functioning of a car.

Signs of a Faulty Coolant Temperature Sensor

Following are the symptoms of a bad coolant temperature sensor:

  1. Check Engine Light
  2. Poor Fuel Economy
  3. Poor Engine Performance
  4. Temperature Gauge Malfunction
  5. Black Smoke From Exhaust

Let us see these symptoms in detail.

Check Engine Light

Check engine light is the first sign you get when something is wrong with your car’s electrical system. When the ECT sensor goes bad it will stop sending temperature readings to the engine control module.

Or it will send faulty readings that are very high or very low compared to normal coolant temperatures. The ECU will set the coolant temperature input to default in either case.

To mark this anomaly in the electrical system of the car check engine light will come upon the dashboard.

Poor Fuel Economy

Engine coolant temperature sensor directly impacts the fuel consumption of your car. It sends the temperature reading to the ECU. According to this temperature the ECU estimates the engine temperature.

If the engine temperature is low, the ECU interprets it as the engine is running cold. So, to warm it up it will increase the supply of fuel. Thus, the poor fuel economy.

Poor Engine Performance

Continuing from the above, if the engine temperature reading sent to the ECU is high, the ECU considers that the engine is running very hot.

So, to cool down the engine, it will slow down the engine cycle and combustion process. Doing this will decrease the amount of fuel being injected into the engine.

As a result, you will experience an underpowered engine and poor performance of your vehicle.

Temperature Gauge Malfunction

When the engine coolant temperature sensor goes bad it will send faulty temperature readings to the ECU. The ECU will in turn determine the wrong engine temperature from it.

This faulty reading is displayed on the temperature gauge on your dashboard. If you experience an unsteady needle in the temperature gauge then something is wrong with the ECT sensor.

Black Smoke From Exhaust

A bad engine coolant temperature sensor can send a faulty signal to the ECU. This can bring an increased amount of fuel to the engine. Excess fuel will remain unburnt and end up in exhaust.

How To Replace a Faulty Coolant Temperature Sensor?

By following the procedure below you can easily replace the faulty ECT sensor yourself:

  1. Make sure that the car is on a level surface and the engine has been off for at least an hour.
  2. Open the hood of your car.
  3. Remove the pressure cap and cap of the overflow reservoir.
  4. On the bottom side of your car, there is a drain plug. Unplug it to drain out all the coolant.
  5. Store the coolant in a safe container.
  6. Now replace the drain plug or plug in the old one. So that you don’t forget it afterward.
  7. The first and foremost task after that is to locate the ECT sensor. It is usually located on the front of the engine, close to the thermostat housing or radiator.
  8. The connection wire between the ECU and the ECT sensor needs to be removed safely.
  9. An ECT sensor is fastened just like a spark plug.
  10. You need to open it with the help of a socket and remove it completely from the engine block.
  11. Clean the hole in which the ECT sensor was located and place the new sensor in it.
  12. Tighten the new sensor in place.
  13. Reattach the wire coming from the ECU.
  14. Cross-check that the drain plug has been tightened.
  15. Now refill the coolant up to the marked level.
  16. Wait for some time and start the engine. Wait till it warms up a bit.
  17. Check for any coolant leaks from the radiator cap and hoses.
  18. Make sure that the check engine light and other warning signs from the dashboard are gone.

Coolant Temperature Sensor Replacement

The replacement of the coolant temperature sensor can cost you between $150 and $200 depending upon the make and model of your car.

If you replace the ECT sensor yourself then it will cost you less than $100.

The sensors like the ECT sensor are not that expensive usually. It is the labor costs that represent the larger portion of the total replacement cost.

Even in the case of an ECT sensor, the price of the sensor itself is only $60 to $90. You have to pay $90 to $110 in labor costs to replace the faulty sensor.


The coolant temperature sensor is one of the most critical components of a car. It measures the temperature of the coolant and communicates it to the ECU which adjusts engine parameters such as spark and ignition timing, fuel injection according to the temperature.

An ECT sensor directly impacts the performance and output of the engine. If it goes bad it can hurt you in the shorter and longer run as well.

The most common symptoms of a bad engine coolant temperature sensor are overheating of the engine, malfunctioning of the temperature gauge, black smoke from the exhaust, low fuel mileage, and poor engine performance.

If you observe any of these signs then a visit to the mechanic is in order. Though replacing a coolant temperature sensor is not a complex task, reconnecting the cooling system can be difficult at times. But you can follow the above procedure to replace the coolant temp sensor yourself.

Frequently Asked Questions

Where is the coolant temperature sensor located?

The engine coolant temperature sensor is usually located near the radiator or thermostat housing. Since its basic function is to regulate the coolant, it is closer to the coolant reservoir.

The size of this sensor is very small due to which you cannot locate it easily. You might need to see through the entire front bay of the engine to find CTS.

Can you drive with a bad coolant temperature sensor?

Yes, you can drive with a bad coolant temperature sensor. When the coolant temperature sensor goes bad the ECU of the car defaults to a static reading.

But it is not recommended to keep driving with a bad engine coolant temperature sensor since it performs critical cooling, heating, and fuel adjustment functions. Its malfunctioning can compromise the working of your entire vehicle.

What is the average lifespan of a coolant temperature sensor?

All sensors including the coolant temperature sensor are designed to last the lifetime of a vehicle. But due to the defects in a car’s electrical system, it may go bad way sooner than that.

Users recommend changing the engine coolant temperature sensor every 50,000 or 60,000 miles. This will have a positive impact on the mileage and performance of your car.

How long can you drive with a failing coolant temperature sensor?

You can drive 1,000 to 2,000 miles with a failing engine coolant temperature sensor. After that, if you observe that symptoms of a faulty sensor are getting severe you should change it immediately.

It is usually not about the sensor but other critical parameters that are being affected due to a failing coolant temp sensor.

Is the cooling fan controlled by the coolant temperature sensor?

No, coolant temperature sensors do not control the cooling fan directly. Rather, they send a signal to the car’s electronic control unit, which then controls the switching of a cooling fan.

A bad coolant temp sensor will send the wrong signal that will compromise the working of the cooling fan as well. This might cause engine overheating.

How long does it take to replace a coolant temperature sensor?

The replacement of a bad coolant temp sensor can take around 2 to 5 hours. The time of replacement depends upon 2 things.:

  1. The skill of the mechanic. If you are doing it yourself it will take 4 to 5 hours. However if a skilled mechanic is doing it, it will take 2 to 3 hours only.
  2. Whether your car was running or not. If the engine of your car is running, then you will have to wait at least 1 hour before you start replacing the bad coolant temp sensor.

Should you drain the cooling system before replacing a coolant temperature sensor?

Yes, it is necessary to drain 1 to 2 quarters of the coolant before replacing an engine coolant temperature sensor.

Since it is located deep inside the lower bay, you need to remove the coolant fluid to access it. You must store it carefully so that it can be used again.

Sign up for our Newsletter

Related Articles