5 Symptoms of a Bad Coolant Temperature Sensor

How do I know if my coolant sensor is bad?

Here are the top 10 Bad Coolant Temperature Sensor Symptoms:

  1. Check engine light
  2. Engine stall
  3. Rough idle
  4. Poor fuel mileage
  5. Car temperature gauge goes up and down
  6. Black smoke from the exhaust
  7. Engine misfire
  8. Engine overheats
  9. Radiator fan not working
  10. Bad engine performance


Engine overheats

If your engine is overheating, it could be a faulty coolant temperature sensor. Sometimes, a faulty sensor sends a permanent cold signal to the engine, causing the engine to overheat.

Other times, a faulty sensor sends a permanent hot signal to the engine, causing the computer to misinterpret the signal and cause the engine to misfire or overheat. In most cases, bad sensors cause an engine to overheat and shut down automatically.

3. Overheating engine

Another symptom of a problem with the coolant temperature sensor is an overheating engine. The coolant temperature sensor can also fail in a manner that causes it to send a permanently hot signal. This can cause the computer to incorrectly compensate for a lean signal, which can result in overheating, and even misfires or engine ping.

What Happens If You Unplug The Coolant Temp Sensor?

Unplugging the engine coolant sensor while the car is running will likely cause the engine to stumble and run rough. The engine light may not come on imminently, but a DTC (Diagnostic Trouble Code) will be logged in the PCM.

You may also notice the engine fan turn on and off intermittently. The engine will run rich, so you’ll smell raw gas, and you may see black smoke from the tailpipe.

Unplugging the engine coolant temperature sensor before starting the engine may result in a no-start, especially likely on a cold morning. Although the PCM will see the sensor is unplugged and has a default fueling strategy for such an event. It is more of a one-size-fits-all approach, and so a cold start in colder temperatures may prevent the engine from starting.

Can Bad Coolant Temperature Sensor Cause Car To Stall?

Symptoms of a bad sensor vary, but a check engine light will be the common denominator. It’s such an important sensor. The engine light will alert the driver as soon as the onboard computer senses a problem.

As you know, the sensor is central to how the engine control modules make decisions about fueling, a flawed reading will cause poor running, and stalling is high on a list of symptoms.

Other common symptoms include:

  • Hard on gas
  • Strong smell of gas
  • Rough running
  • Hesitation on acceleration
  • High or low idle
  • Misfiring
  • Black smoke
  • Bad smell from the muffler
  • Overheating engine
  • Fan running constant
  • HVAC blows cool air


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.

Final Thought

Your vehicle engine has to stay within a specific temp. If there’s any disruption to this, it will get overheat and probably cause catastrophic engine damages. To prevent the engine from overheating, car manufacturers design cars to use antifreeze that runs through the radiator to keep the engine cool. The coolant needs to be measured to ensure the vehicle engine runs at average operating temperature continually. Hence, the need for a coolant temp sensor (CTS).

Coolant temp sensor failure can result in specific engine problems that can cause a blown head gasket or even damage an aluminum engine block in a severe case. Therefore, you should always watch out for symptoms of a bad coolant temp sensor and fix it using the simple steps above or contact your mechanic for the replacement.

Decreased Mileage

Though this is an ambiguous indication, suggesting a medley of problems, it could be telling of a coolant temperature problem if it occurs before the other listed problems. It is a concomitant effect of a faulty coolant temperature sensor to cause a drop in your fuel economy. 

The bad coolant sensor feeds an inaccurate reading to the system, leading it to miscalculate the timing and amount for the deployment of fuel to the engine.

If the system continues to receive amiss signals from the broken sensor, it will interpret the engine to be going cold, thereby, altering its way of functioning and injecting more fuel into the engine than needed.

This cycle will cause the engine to deliver poor fuel economy and more importantly, will lose its efficiency in the long run.

What Does the Coolant Temperature Sensor Do?

Basically, your engine coolant temperature sensor (CTS or ECTS) monitors the temperature of the coolant circulating through your car’s engine. In turn, this gives an idea of how hot your engine is. If your engine is overheating, it’s up to the coolant temperature sensor to pick up on that information and trigger the dashboard light, as well as any other responses your car is set to carry out.

Many modern cars will automatically shut off the engine once it reaches a certain temperature to protect it from excessive damage. If your coolant temperature sensor isn’t doing its job properly, the engine may not receive the signal to shut off  and you might find yourself facing serious engine damage.

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Engine Coolant Temperature Sensor Replacement Cost

Best places to order parts?  See: 19 Best Online Auto Parts Stores

The cost associated with coolant temperature senso

The cost associated with coolant temperature sensor replacement can vary significantly from one model of vehicle to the next.

While the coolant temperature sensors found on many vehicles are relatively simple to replace, others can be quite difficult to access. The actual cost associated with the purchase of a coolant temperature sensor can also vary by vehicle.

On average, the cost of a coolant temperature sensor ranges from $20-$80, while the cost of labor ranges from $50-$250. In total, one could expect to pay $70-$330 to have their engine coolant temperature sensor replaced.

Coolant Temperature Sensor Location

The engine coolant temperature sensor is usually l

The engine coolant temperature sensor is usually located on the engine block or cylinder head. It is often installed on a plastic hose on the coolant’s inlet.

Different brands and car manufacturers have different ways of placing the coolant temperature sensor depending on the car design.

Some vehicles may have more than one temperature sensor, as sometimes different sensors are used to signal to the dashboard, cooling fan control, and the control unit of your engine system.

With two sensors, we usually consider sending signals to the control unit as the coolant temperature sensor.

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.

Diagnosing a Malfunctioning Coolant Temperature Sensor

Diagnosing should be the job of a certified technician. However, this shouldn’t stop you from examining the engine component if you suspect it is malfunctioning.

You may have to get your hands on a repair manual for the model of your vehicle. This will enable you to find the measurement values that a functioning coolant temperature switch should exhibit.

Check the service manual of your vehicle to locate your coolant temperature sensor. When you set eyes on it, disconnect the connector plugs.

If your sensor has two pins, determine the ohm measure between the pins. Then compare the value with the correct ohm-value – at a given temperature – in your repair manual. If the value does not tally, it means you need to replace the coolant temperature sensor.

If the values add up or tally, consider checking the connectors and wirings between the sensor and the engine’s control module.

You can also make use of an OBD2 Scanner to look for related trouble codes. Check the live data to see the temperature of the sensor. If the temperature is off the range, check the wirings of the sensor or replace the engine component.

Check Engine Light

Most modern vehicles have a coolant level scanner located on the coolant reservoir that monitors the temperatures and triggers a dashboard check light or warning sign when the coolant level gets critically low.

However, it can also be a sign of a broken coolant temperature sensor if the actual coolant levels are in the normal range. Regardless the check engine light is an important signal and shouldn’t be ignored. It is advised totake your car for a detailed inspection in order to avoid any hefty engine damage.


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