7 Symptoms of a Bad Coolant Temp Sensor & Cost to Fix

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symptoms of a bad coolant temp sensor

People often confuse a bad coolant temperature sensor with a lousy manifold absolute pressure sensor or bad fuel rail pressure sensor. The reason behind it is the similar kind of signs these sensors give off before going out.

You can only get to the root cause of the problems if you keep a keen eye on the symptoms.

The extent of symptoms also differs between various sensors. For example, the majority of sensors cause engine overheating when they go bad. But coolant temperature sensor will cause most problems regarding an overheated engine.

Since the coolant temperature sensor is primarily responsible for keeping the engine’s temperature under check. At the same time, MAP or fuel rail pressure sensors have different primary functions.

Following are the most common symptoms of a bad coolant temp sensor:

  1. Check Engine Light
  2. Engine Overheating
  3. Poor Fuel Economy
  4. Poor Engine Performance
  5. Broken Water Pump
  6. Control of Cooling Fan
  7. Black Smoke From Exhaust Pipe

This article will explain the signs of a bad engine coolant temperature sensor in great detail.

What is a Coolant Temperature Sensor?

An engine coolant temperature sensor is also known as CTS or ECT sensor, as it is evident from its name that it measures the temperature of the engine coolant. In addition to this function, it also measures the overall engine temperature.

Overall engine temperature is measured by estimating the amount of heat the engine gives off when it is running.

Keeping a check on coolant temperature is very important. The coolant is the soul of the cooling system of a car. If the coolant becomes too hot, it can cause many problems for your engine.

Coolant is converted into steam when it starts boiling. Instead of liquid coolant, this steam becomes the working fluid of your cooling system. As a result, its work is compromised.

How Does A Coolant Temperature Sensor Work?

An engine coolant temperature sensor is made up of resistors. When the engine starts running, the engine control module sends a voltage signal to the coolant temp sensor.

The amount of resistance offered by the coolant temp sensor is calibrated against readings on a temperature scale. This way, coolant temperature is measured.

The temperature of the coolant also gives an estimate of the amount of heat generated in the engine.

This engine temperature is displayed on the temperature gauge in your dashboard.

Symptoms of a Bad Coolant Temp Sensor

Following are the significant symptoms of a bad coolant temp sensor:

  1. Check Engine Light
  2. Engine Overheating
  3. Poor Fuel Economy
  4. Poor Engine Performance
  5. Broken Water Pump
  6. Control of Cooling Fan
  7. Black Smoke From Exhaust Pipe

Let us take a closer look at the symptoms of lousy coolant temperature sensors.

1. Check Engine Light

If anything goes wrong with your car’s electrical system, then check engine light is the first thing to appear on your dashboard.

I’m personally not a great fan of check engine light because it tells you that there is a problem. But it never tells you what that problem is. For that, you need to keep an eye on other signs as well.

Even in the case of a faulty coolant temp sensor, a check engine light will pop up on your dashboard. But you would need to figure out for yourself what is the actual problem.

2. Engine Overheating

Any problem with your car’s coolant system will cause the engine to overheat. Since the primary function of the cooling system is to keep the engine temperature under check.

If a critical component like the coolant temperature sensor goes out, the engine will overheat.

The engine coolant temperature sensor communicates the coolant temperature to the ECU. Wrong communication can lead to engine overheating.

3. Poor Fuel Economy

Based on coolant temperature sensor input, the ECU decides the fuel injection and fuel mixture ratio. In the case of a faulty sensor, a car’s fuel consumption can increase up to many folds.

How does this work? When you have a faulty coolant temperature sensor, the chances are that it will be reading coolant temperature that is lower than the actual coolant temperature at the time.

In which case, the ECU decides to inject more fuel to warm up the engine quickly. Since more fuel would produce more heat, the engine temperature will reach the optimum level sooner.

As a result, more fuel is consumed than usual. This is how a bad coolant temperature sensor increases fuel consumption.

4. Poor Engine Performance

A bad engine coolant temperature sensor can also cause the poor performance of your vehicle.

A bad engine coolant temperature sensor can send a faulty signal of low coolant temperature. It can also send the wrong signal of high coolant temperature when it is not.

So, when the ECU receives the signal from the coolant temp sensor that the coolant temperature is high, it takes that the engine is getting overheated.

This case reduces the fuel injection ratio to cool down the engine immediately. Since lesser fuel will make the engine run leaner, it helps the engine reach optimal performance.

But the engine performance settings are disturbed, and your vehicle’s engine feels powerless while accelerating.

5. Broken Water Pump

A bad engine coolant temperature sensor can also give you a broken water pump. If your water pump has gone bad before reaching its expected life, then it is a sign that something is wrong with the coolant sensor.

It happens when the faulty coolant temperature sensor reads coolant temperature higher than actual. In this case, the engine control unit takes corrective measures.

Not only will it increase the fuel injection, but it will also increase the water pump’s speed so that it pumps more and more coolant to cool down the engine at the earliest.

This puts unnecessary strain on the water pump, due to which it can go lousy way sooner before reaching its actual life.

6. Cooling Fan Control

Though the coolant temperature sensor is not directly responsible for controlling the cooling fan, its input decides when to switch it on and when to switch it off.

When the coolant temp sensor malfunctions, it might report a lower or higher coolant temperature than actual.

Based on this faulty reading, your car’s ECU will adjust the speed and switching patterns of the cooling fan.

So, when you experience that the cooling fan is on when it is not needed and is off when you need it the most, the coolant temp sensor has gone bad.

7. Black Smoke From Exhaust Pipe

A faulty coolant temperature sensor can disturb the fuel mixture ratio of your engine.

An optimum amount of fuel is required for the complete combustion process. If a proper fuel supply is not maintained, the combustion process remains incomplete.

A faulty signal from the coolant temperature sensor can either bring in lower or higher amounts of fuel.

In the latter case, extra fuel will stay unburnt and remain in the combustion chamber. This unburnt fuel ends up in the exhaust of your car. It will cause black smoke to come out of your car’s muffler. You could also get fuel droplets in the exhaust.

In either case, you need to worry about the health of your coolant temperature sensor.

How To Replace a Bad Coolant Temp Sensor?

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

Prepare the Working Area:

  • Turn off the engine at least 1 hour before you start working.
  • Park your car on a level surface.
  • Open the hood of your vehicle.
  • Remove the pressure cap and cap of the overflow reservoir.

Drain Out the Old Coolant:

  • Underneath your car is the drain plug of the cooling system.
  • Unplug it to drain out all the coolant.
  • Store the coolant in a safe container.
  • Now replace the drain plug or plug in the old one. So that you don’t forget it afterward.

Locate ECT Sensor and Remove it:

  • Locate the ECT sensor. It is located on the front of the engine, close to the thermostat housing or radiator.
  • Remove the connection wire between the ECU and the ECT sensor safely.
  • An ECT sensor is fastened just like a spark plug.
  • You need to open it with the help of a socket and remove it entirely from the engine block.

Install New CTS & Reattach Wires:

  • Clean the hole of the ECT sensor from dirt and debris.
  • Then place the new sensor in it.
  • Tighten the new sensor in place.
  • Reattach the wire coming from the ECU.

Refill the Coolant:

  • Again check that the drain plug is tight.
  • Now refill the coolant up to the marked level.

Start the Vehicle:

  • Give some time for the coolant to settle. Then start the engine.
  • Wait till it warms up a bit.
  • Check for any coolant leaks from the coolant system.
  • Ensure that the check engine light and other warning signs from the dashboard are gone.

Engine Coolant Temperature Sensor Replacement Cost

The total cost for replacing a bad coolant temp sensor is between $150 and $200. The price varies with respect to the workshop you are visiting, the skill of the mechanic, and the country in which you live.

The sensor itself is not very expensive. You can get a coolant temperature sensor for $60 to $90 at any auto shop. But the labor is going to cost you around $100.

You can try to replace the faulty coolant temperature sensor yourself. It will save you the labor cost.

The process is not that complex. You need to be careful while resetting the cooling system.


Engine coolant temperature sensor is probably one of the most critical sensors in your car. If it malfunctions, it will directly impact the fuel efficiency, fuel mixture ratio, and consequently your vehicle’s performance.

Due to changes in the fuel mixture ratio of the engine, you might start getting black smoke in from the muffler. It is due to unburnt fuel during the combustion process.

When you get a check engine light and your engine is overheating more than usual, it is a clear sign that something is wrong with your vehicle’s cooling system.

A bad coolant temperature sensor puts strain on other cooling system components. As a result of which they can go bad as well. The water pump is on the top of that list.

It would be best to keep an eye on the signs of a bad coolant temperature sensor. Once you have established that it has gone bad, you must visit the mechanic immediately and replace it.

Frequently Asked Questions

What is the average life span of a coolant temperature sensor?

All sensors are designed to last the lifespan of your vehicle. But usually, this is not the case. A coolant temperature sensor can go bad around 60,000 miles mark.

Malfunctioning components of the electrical system can cause its early failure. Or it can just fail due to unfavorable external factors.

What parameters are controlled by the coolant temperature sensors?

The coolant temperature sensor controls many critical parameters of the engine. These parameters include fuel economy, air-fuel mixture ratio, fuel injection, spark, and ignition timing.

How long can you drive with a bad coolant temp sensor?

You can drive 1,000 to 2,000 miles with a bad coolant temperature sensor. When the coolant temperature sensor goes bad, the engine management system sensor defaults coolant temperature to a static reading.

This default setting will aid in keeping the coolant temp under check. But this function is only available in modern cars. With old cars, you do not have this luxury. So, if the symptoms are severe, I would recommend not even driving a single mile.

Can a bad coolant temp sensor put your vehicle’s engine in limp mode?

No, a bad coolant temperature sensor is unlikely to put your car in limp mode. When the coolant sensor goes bad engine management sensor defaults coolant temp to a static reading.

Hence, the working of the car stays normal. The ECU does not put your car into limp mode.

Where is a coolant temperature sensor located?

The coolant temperature sensor is located in the front bay of your engine. It is near to the thermostat housing or radiator. The coolant temperature sensors are tiny. So, you might need to look closely to find it.

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