Top 10 Bad Coolant Temperature Sensor Symptoms

In this article, we are going to discuss the most common bad coolant temperature sensor symptoms. It remains one of the most vital engine management sensors. Anyone who has ever worked on a car can tell you how important engine temperature is.

It’s a safe bet that the vast majority of drivers are likely unfamiliar with their coolant temperature sensor. It may not be the first thing they think of when they hear the word “engine,” but it should be. Because once the coolant temperature sensor fails, you’re going to be in for a very unpleasant ride.

Bad Coolant Temperature Sensor Symptoms
Car temperature gauge Wikimedia Commons

What is the coolant temperature sensor?

The coolant temperature sensor is what keeps your engine running smoothly and efficiently. The sensor acts as the thermometer of your engine, helping to maintain your car’s temperature. Depending on the CTS readings, the control unit adapts the injection time and firing angle to the operating conditions.

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

Check engine light

The sensors measure how hot the engine is running and if it’s close to overheating. If the computer senses that the engine is overheating it will show a check engine light. A bad or malfunctioning thermostat will result in a trouble code generated by the computer system. This code triggers the Check Engine warning light on the dashboard.

Here are some of the most common diagnostic trouble codes (DTC) for “Engine Coolant Temperature (ECT) problems:

  1. P0115 Code: Engine Coolant Temperature Sensor Circuit Malfunction
  2. P0117 Code: Engine Coolant Temperature Sensor 1 Circuit Low
  3. P0118 Code: Engine Coolant Temperature Circuit High Input
  4. Engine stall or no start

Code readers and scanners can pinpoint an issue with your vehicle without having to take it to an expensive mechanic. We found the Innova 5610 scan toolset to be a worthwhile investment for any DIY enthusiast. You can check the latest price on Amazon.

Rough idle

If the engine is cold and the sensor reads hot, the computer adjusts its fuel delivery accordingly and you may experience stumble, hesitation, and rough idle at startup.

The opposite may happen if it read cold all the time when it should be reading hot. You’ll have to check this with a scan tool if your vehicle has one or wait for a code to set to see if there is an issue with this sensor.

If the coolant sensor is working, but it is out of range, it will not set any codes. It may not be bad, but just reading wrong. This could be caused by a bad sensor or wiring in the harness.

Poor fuel mileage

Poor fuel mileage can be a symptom of the bad coolant temperature sensor. When the engine is cold, there is more fuel delivered; and longer spark duration. As the engine warms up; and the coolant temperature sensor senses a temperature rise; it sends a signal to the PCM to reduce the amount of fuel delivered for each pulse. It also shortens the spark duration, so less fuel is burned.

When you have a failed coolant temperature sensor; it can send a permanently cold signal to the PCM (Powertrain Control Module). The result of this is that the engine will run rich (too much fuel); and will not warm up as quickly as it should. The combination of these factors will drastically affect your gas mileage. But many other things could be wrong with your car, too.

Car temperature gauge goes up and down

If the car temperature gauge goes up and down randomly, it can be a sign that you have a bad engine coolant temperature sensor. The sensor monitors the temperature of the coolant and sends this information to the computer. When the computer doesn’t receive accurate information, it may not be able to use its fuel or timing control functions properly.

If this happens, it can cause the engine to not run as well or at all. If the sensor fails, it may also fail in a way that sends a signal that is always high or low, which can cause the same types of problems.

Black smoke from the exhaust

A faulty coolant temperature sensor can prevent the engine from starting, but in other cases, the engine will start, but then it may crank too fast or too slow. If the engine does start, you may notice heavy black smoke from your tailpipe.

The black smoke is a result of an overly rich air-fuel mix in the engine caused by incorrect sensor readings. If you’ve noticed that your car’s exhaust pipe is producing black smoke anytime you accelerate, then it’s probably a good idea to have its coolant temperature sensor checked out as soon as possible.

Engine misfire

A bad engine coolant temperature sensor can have a huge effect on your car’s performance. The sensor is used to measure the temperature of the coolant and send it to the ECM. The ECM then uses this information to adjust the fuel ratio, ignition timing, etc.

If the engine of your car has been misfiring recently, it is time you should have it investigated by a mechanic. The problem of engine misfire can be caused by a bad coolant temperature sensor. This problem has to be solved on time to avoid irreparable damage to the engine.

You may also want to check out our article on how to fix engine misfires.

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.

Radiator fan not working

The radiator fan is responsible for helping to provide airflow into and out of the radiator to assist with cooling. A broken coolant temperature sensor can also make the radiator fan not work as well. When this sensor fails it may cause overheating in the process.

This sensor may also relate to other different symptoms. As the control system receives the temperature from the CTS, it may trigger the cooling fan to either shut off or turn on.

Bad engine performance

If you feel that the engine is not performing as it has been before, there may be a possibility that the engine coolant temperature sensor is to blame. The ECT sensor can have a direct impact on the air-fuel mixture.

We have discussed earlier in this article, how a bad air-fuel mixture can cause the engine performance to drop drastically. Hence if you feel that the engine performance is not what it has been before, it may be due to a faulty engine coolant temperature sensor.

What causes a coolant temperature sensor to go bad?

Three things cause a coolant temperature sensor to go bad including:

  1. Poor sensor connection
  2. Contamination in the coolant
  3. Wear and tear

One of the common problems is when the sensor has a poor connection inside or in the connector. This causes interruptions in the signal to the PCM and the PCM sets the fault. In some vehicles, symptoms of this problem show up as erratic readings of the temperature gauge.

A second problem occurs when there is some kind of contamination in the coolant. On some vehicles, there is an air bleed screw on top of the thermostat housing to let air escape from the system after a repair. If this bleed screw isn’t closed properly, it will allow dirt and debris into the system, which can clog up the sensor port.

The third cause of failure is when the coolant temperature sensor itself fails internally or becomes contaminated due to age or overheating. The most common failure mode is when a blown head gasket allows coolant to leak into one or more cylinders and then get burned as combustion gas. 

This leaves behind a black residue that can contaminate the coolant, which eventually works its way into all parts of the cooling system, including any sensors installed in it.

Is it safe to drive with a bad coolant sensor?

The coolant temperature sensor is a critical component used by the engine management system. It directly affects the cooling and fueling of the engine and therefore affects how the engine performs.

It is possible to drive a vehicle with a faulty coolant temperature sensor as the management system defaults to a static reading. 

This may be adequate for a while but will eventually lead to the malfunction of certain systems, such as the cooling system.

How do you test a coolant temperature sensor?

This video shows you how to test a coolant temperature sensor. Here are some things that you will need:

  1. Multimeter
  2. Cup
  3. Water


It is worth noting that the top bad coolant temperature sensor symptoms are not just limited to the vehicle in the article. The most common sign of this failure is the check engine light going on, but other symptoms include stalling, rough idle, and poor fuel economy.

Furthermore, driving with a bad coolant temperature sensor can lead to a vehicle overheating. While this article does not deal with what occurs when not fixed as quickly as possible, we do recommend having the problem diagnosed as soon as possible before it causes any significant damage.

Check out other guides on car sensors:

Driving With Bad Oil Pressure Sensor – Why is it Unsafe?

How To Tell Which ABS Sensor Is Bad: 2 Simple DIY Methods

How Long Can You Drive a Car With a Bad Speed Sensor?

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