Car Repair and Maintenance

Bad Coolant Temp Sensor Symptoms [and Replacement Cost]

Bad coolant temp sensor symptoms

If you suspect abnormalities, you want to tell the bad coolant temp sensor symptoms in your car. Note also that a bad coolant temperature sensor is often mistaken for a defective manifold absolute pressure sensor or a bad fuel rail pressure sensor.

For your car to operate at its best, an engine coolant temperature sensor is necessary. The engine control module’s coolant temperature is measured via the engine coolant temperature sensor.

Additionally, the severity of symptoms of coolant temp varies across different sensors. For instance, when sensors malfunction, the majority of them lead to engine overheating. The coolant temperature sensor, however, will be the main source of issues with an overheated engine.

In this article, we will discuss the possible symptoms of bad coolant temp sensor and how to tell if your coolant temp sensor is bad.

Bad coolant temp sensor symptoms

What is a coolant temperature sensor?

Since it is clear from its name that an engine coolant temperature sensor detects engine coolant temperature, it is sometimes referred to as a CTS or ECT sensor. It also gauges the engine’s total temperature in addition to performing this job.

By calculating how much heat the operating engine emits, the total engine temperature is determined. It’s crucial to monitor the coolant temperature. The coolant is the lifeblood of a car’s cooling system. Your engine can experience a variety of issues if the coolant becomes too hot.

When coolant begins to boil, steam is produced. This steam serves as the cooling system’s operating fluid in place of liquid coolant. This compromises its ability to do its job.

How does a coolant temperature sensor work?

Resistors make up an engine coolant temperature sensor. The coolant temperature sensor receives a voltage signal from the engine control module when the engine begins.

The coolant temperature sensor’s level of resistance is calibrated using temperature scale readings. Coolant temperature is monitored in this manner.

The quantity of heat produced by the engine may also be estimated from the coolant’s temperature. The temperature indicator on your dashboard shows the engine temperature.

Bad coolant temp sensor symptoms

Below are common signs and symptoms that you have a bad coolant temp sensor:

1. Poor fuel economy

The ECU selects the fuel injection and fuel mixture ratio based on data from the coolant temperature sensor. A malfunctioning sensor might result in a car’s fuel usage rising significantly.

How does it function? The likelihood is that a malfunctioning coolant temperature sensor will provide a number that is lower than the actual coolant temperature at the moment.

In such circumstances, the ECU chooses to increase the fuel injection to hasten the engine’s warming up. More gasoline would generate more heat; therefore, the engine temperature would rise to its ideal level faster.

As a result, more fuel is used than normal. This is how a malfunctioning coolant temperature sensor increases gasoline use.

2. Check engine light

The check engine light will likely appear on your dashboard as one of the first bad coolant temp sensor symptoms.

The check engine light will come on if the computer detects any issue with the sensor’s circuit, signaling that the vehicle needs to be inspected.

It’s time to check the issue codes using an OBD2 scanner if you see a check engine light on your dashboard.

3. Overheated engine

The engine will overheat if there is an issue with the coolant system in your car. Since maintaining a stable engine temperature is the cooling system’s principal duty. The engine will overheat if a crucial component, such as the coolant temperature sensor, fails. The ECU receives coolant temperature information from the engine coolant temperature sensor. Before you ignore that overheating engine, know that severely overheating engine can cause the cylinder walls or the pistons themselves to warp, bend and deform—Hansonsubaru confirms.

4. Broken water pump

A damaged water pump might also result from a bad engine coolant temperature sensor. It is a clue that there is a problem with the coolant sensor if your water pump fails before it should have.

This occurs when a defective coolant temperature sensor reports a greater coolant temperature than is the case. In this instance, the engine control unit will fix this issue.

It will speed up the water pump as well as the fuel injection, pumping more and more coolant to the engine to cool it down as soon as possible. The water pump is placed under needless strain as a result, and it may fail far earlier than its real-life expectancy.

5. Electric cooling fans not turning on

The electric cooling fans in certain cars are controlled by the engine coolant temperature sensor. For the fans, dashboard gauge, and engine management in the majority of cars, there are two different temperature sensors.

A bad coolant temp sensor, however, might prevent your fans from turning on if your car just has one sensor.

6. Poor engine performance

Your car may perform poorly due to a faulty engine coolant temperature sensor.

An incorrect indication of low coolant temperature may be sent by an engine coolant temperature sensor. Additionally, it may miscommunicate a high coolant temperature when none exists.

Therefore, the engine is assumed to be overheating when the ECU gets a signal from the coolant temperature sensor indicating that the coolant temperature is high.

In this situation, the fuel injection ratio is lowered to quickly cool the engine. Less gasoline helps an engine operate more efficiently since it makes it run leaner. However, the engine performance settings are messed up, and accelerating with your car seems weak.

7. Black smoke coming from the exhaust pipe

The fuel mixture ratio in your engine might be thrown off by a bad coolant temperature sensor. For the whole combustion process, the right quantity of fuel must be used. The combustion process is not completed if the appropriate fuel supply is not kept up.

Lower or higher fuel levels may be introduced depending on the quality of the coolant temperature sensor’s output. In the latter scenario, additional fuel will remain in the combustion chamber unburned.

The unburned gasoline ends up in your car’s exhaust. It will result in the exhaust pipe of your car producing black smoke. Additionally, gasoline vapors might end up in the exhaust.

You should be concerned about the condition of your coolant temperature sensor in any scenario.

8. Hard start condition

This is one of the ways how to tell if your coolant temp sensor is bad. With the quantity of gasoline put into the engine, the starting moment of a car is quite important. Your car can be difficult or impossible to start if the air-fuel combination is off.

9. Poor idling

A bad coolant temp sensor will cause the fuel mixture to change. When the car is moving slowly, this will result in the engine shaking or vibrating, as well as other power losses and odd behaviors.

When idle, when the engine is most vulnerable to improper air-fuel ratios, you could detect a problem with your engine coolant temperature sensor.

10. Irregular temp readings

A failed or subpar coolant temperature sensor may be the cause of temperature variations in your temperature gauge or if it consistently reads lower or higher than normal while the engine is operating. Especially if the engine doesn’t warm up as it should, you need to call a repair to identify and locate the cooling system issue.

Even while the coolant temperature may be lower than the surrounding air temperature, it may nevertheless give the auto-computer false signals.

However, bear in mind that in addition to the defective coolant temperature sensor, several additional factors might cause the temperature to fluctuate or vary irregularly, including cylinder head leaks, low coolant levels, worn-out radiator fans, and damaged pressure caps.

Coolant temperature sensor location

The engine block or cylinder head is often where the engine coolant temperature sensor is found. On the coolant intake, it is often mounted on a plastic hose.

Depending on the architecture of the car, various brands and automakers have varied placement options for the coolant temperature sensor.

As various sensors are sometimes utilized to communicate to the dashboard, cooling fan control, and the control unit of your engine system, certain cars may have more than one temperature sensor.

Coolant temperature sensor replacement cost

Depending on the vehicle type and labor expenses, the cost to repair a coolant temperature sensor ranges from $50 to $250. In addition to the labor expense, a coolant temperature sensor may be purchased for $30 to $100.

The coolant temperature sensor is often relatively affordable, costing approximately $40 for a good one. The replacement is often also pretty simple, with the exception that you may need to drain the whole amount of coolant from the engine and refill it, which requires tough air removal from the coolant system.

In contrast, tapping out the coolant is often unnecessary if you are changing the sensor quickly, although doing so needs some expertise. When doing this kind of maintenance, always make sure the coolant temperature is low.

How to diagnose a faulty engine coolant temperature sensor?

To determine what measurement results to anticipate from the coolant temperature sensor, you will need a repair manual for your specific car model.

  1. Plug in an OBD2 scanner and search for relevant error codes. To see the temperature recorded by the sensor, view the live data. Check the wirings and replace the sensor if it is far beyond the range.
  2. Use the service handbook to locate the coolant temperature sensor in your car.
  3. Locate the connecting plugs for the coolant temperature sensor.
  4. If your coolant temperature sensor has two pins, you might try measuring the resistance between them.
  5. Consult your repair manual to get the appropriate ohm value for a certain temperature.
  6. Replace the sensor if the value is incorrect.
  7. If everything seems to be in order, examine the cabling and connections that link the engine control module to the coolant temperature sensor.


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

The kind of car you have and your level of expertise will determine how long it takes to repair a coolant temperature sensor. However, replacing the majority of coolant temperature sensors takes between 30 and 60 minutes.

Will the engine coolant temperature sensor keep a car from starting?

Yes, a faulty coolant temperature sensor might make it impossible to start the engine. For instance, if the sensor informs the ECM that it is warm outdoors but it is extremely cold outside, the engine won’t start.

Do you have to drain the coolant to replace the temperature sensor?

Yes, you must drain the coolant from most car types to replace the temperature sensor. When the sensor is quite high up on the engine of particular car types, it is occasionally possible to swiftly swap to a new sensor instead of emptying the whole system. When servicing your car’s engine, make sure the coolant is cold.

Is there a fuse for the coolant temperature sensor?

No, the coolant temperature sensor itself probably won’t have a fuse. The engine control module, which provides the signal to the coolant temperature sensor, has a fuse that you can locate.


Poor fuel economy, failure of cooling fans to operate, problems with engine performance, an overheated engine, and a check engine light on your dashboard are the most typical signs of a bad coolant temperature sensor.

Take your car in as soon as you can for a checkup if you see any of these signs. Your car may overheat if the coolant temperature sensor is defective. If you think the coolant temperature sensor is broken, it’s crucial to be aware of the symptoms and get it replaced as soon as you can since this might result in major engine damage.

The good news is that most car models allow for the relatively simple and inexpensive replacement of a coolant temperature sensor, so you don’t need to be a mechanic to do the task. However, you must exercise caution and ensure that the engine cools down completely after each use.

Latest posts by Bernard Juchli (see all)

Bernard Juchli

Bernard Juchli is an experienced racer, mechanic and team owner who trusts Avon Tyres.Bernard is the lead driver and force behind his Big Dog Garage Race Team. He is the General Manager and Chief Mechanic of Jay Leno’s Garage. Bernard and his crew of seven are responsible for all repairs, restoration and fabrication of Jay’s incredible automobile and motorcycle collection.

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