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Coolant Temperature Sensor: How it Works, Problems, Symptoms, and Solutions!

Coolant Temperature Sensors

The coolant temperature sensor helps keep the engine at its optimal operation temperature and prevents it from overheating. It is the one that measures the liquid coolant’s temperature. The collected data will then be sent to the car’s computer, which it will use to determine whether it will continue to operate or will adjust certain engine functions to keep the engine within its ideal temperature range. The coolant temperature sensor can also become faulty which give rise to some problems with symptoms such as engine overheating, check engine light, poor fuel economy, rough idle, poor engine performance, and many more. 

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Coolant Temperature Sensor: How do coolant temperature sensors work?


The car’s coolant temperature sensor, also known as the engine coolant temperature sensor, operates to measure the coolant or antifreeze mix temperature in the cooling system and gives data of how much the engine is giving off heat. This sensor works with the Engine Control Unit or the ECU to monitor the temperature of the coolant to ensure that the engine operates at its optimum temperature. 


A coolant temperature sensor is usually a negative temperature coefficient thermistor. This means that its electrical resistance will decrease every time the temperature increases. The coolant temperature sensor’s tip sticks out into one of the passages of the cooling system and is dipped in the coolant. 


The coolant temperature sensors can vary from car to car. There are many cars that are equipped with more than one coolant temperature sensor. The primary sensor in most cars is usually installed close to the thermostat in the cylinder head, block, or sometimes, on the thermostat housing. The second sensor on the other hand, can be found in another area of the engine or the radiator. 


There are also some cars that are installed with a cylinder head temperature sensor instead of a coolant temperature sensor – sometimes, both are installed. The cylinder head temperature sensor works just like the CTS but it is not dipped in the coolant and measures the cylinder head metal’s temperature. This kind of sensor can still get the correct measure of the engine temperature even with coolant loss which can help prevent the engine from overheating.  


The engine control unit sends a regulated or reference voltage, usually around 5 volt, to the coolant temperature sensor to obtain an accurate reading of the engine temperature. It also constantly monitors the coolant temperature sensor signal. Based on the signals collected, the engine control unit adjusts the performance of the engine, the fuel mix, the ignition timing, and operates or controls the electric radiator fans, switching it on or off, when the temperature gets to a predetermined level. 


The signals or data collected will also be used to provide an accurate reading of the temperature of the engine to a dashboard gauge. If the engine control unit can get any signal from the coolant temperature sensor, the ECU will set off the check engine light and will store in its memory the related trouble code.  


Coolant Temperature Sensor: What causes a coolant temperature sensor to fail?


The coolant temperature sensor can go bad over time like any other parts of a car. When the CTS becomes faulty, it can cause a range of problems. One of the common problems of the coolant temperature sensor is when it suddenly has poor connection inside the connector or in the wiring harness. When this happens, there will be signal interruptions to the engine control unit and the ECU sets the fault. This can lead to erratic readings of the temperature gauge in some cars. Other cars may have an engine that runs in fail-safe mode


Another thing that can cause some sensor-related faults is corrosion at the sensor terminals or the connectors. There are instances where water contamination in the sensor connectors were reported and it has caused the trouble codes P1285, P1299, or P0128 to occur. To correct the problem, the connectors and the sensor will have to be cleaned or replaced depending on the severity of the corrosion. 


You will know if you have a failing coolant temperature sensor if you encounter other symptoms aside from engine overheating and an illuminating check engine light which include:


  • Irregular temperature reading 


Fluctuating engine temperature that seems like it is lower or higher than usual when you are driving can be a sign that your coolant temperature is failing. An example is when you notice that your engine is not warming up as it’s supposed to. Although it could mean that the temperature outside is cooler, it is also possible that the sensor is sending inaccurate temperature data which is lower than it actually is.   


  • Reduced fuel economy


The coolant temperature sensor is also being used to make sure that the right mixture of fuel is used. A faulty CTS can send inaccurate data or signals that can cause the computer to throw off the fuel and timing calculations. The CTS can send a permanently cold signal to the computer which can cause it to think that the engine is cold when it is not. This reduces the fuel economy since the engine will be supplied with more fuel than necessary. 


  • Black smoke from the exhaust


If you notice a black smoke coming from the exhaust of your car, it can be an indication that you have a faulty coolant temperature sensor. When the CTS is failing, it could send a cold signal to the engine control unit and it can cause it to unnecessarily enrich the fuel mixture.  When the fuel mixture gets too rich, it will not be burned sufficiently in the combustion chamber but will be burned up in the exhaust pipe instead, causing black smoke to come out of your exhaust. If the black smoke becomes too thick, your car might become unsafe to drive. 


  • Poor engine performance


A bad air to fuel mixture can cause the performance of your engine to drop drastically. Bad fuel mixture can be caused by a faulty coolant temperature sensor sending inaccurate signals to the car’s computer. 


  • Rough idle


A bad coolant temperature sensor can cause the ECU to adjust the fuel mixture. Since the engine is very sensitive to incorrect air to fuel mixture when idling, it will cause the engine to vibrate or shake when it is idling or is running at slow speed. It could also result in other power losses and other issues. 


Coolant Temperature Sensor: How much does it cost to replace a coolant temperature sensor?


If your car has been diagnosed with a faulty coolant temperature sensor, you need to have it repaired as soon as possible and avoid driving it. Driving a car with an engine cooling system that is faulty can cause a lot of problems such as cylinder head gasket failure, overheating, cylinder head warping, or engine block failure that can be too expensive to fix. Experts recommend not to drive a car that has issues with its cooling system, more so for cars that have modern engine casting materials. 


To repair a faulty coolant temperature sensor, it often needs to be replaced. The engine coolant temperature sensor replacement has an average cost of between $135 and $155. Labor costs can go between $83 and $105 while the parts can be bought for around $50. 


How often does the coolant temperature sensor need to be replaced? Failing coolant temperature is something that usually happens but it can happen more frequently than usual if you fail to maintain your car’s engine cooling system. A coolant temperature sensor can last up to 100,000 miles and when it hits that mile mark, issues usually start to occur that would warrant a replacement. You might have to replace it earlier if your CTS has not been properly maintained and corrosions and other problems start to appear. 


Coolant Temperature Sensor: How to test the coolant temperature sensor? 


Checking the resistance.


Testing the coolant temperature sensor means measuring the resistance of the sensor and it can be done in several ways. One of the ways is to measure its resistance at different engine temperatures and by comparing the readings to your service manual’s specifications. 


This is done by measuring the coolant temperature sensor when it is disconnected from the circuit. If you measure the resistance of any of the car’s electrical components still connected to the circuit, you will likely get an inaccurate measurement. You need to know the correct specifications of your resistance from your service manual to make sure that you can compare it to your readings. Check the resistance when the engine is cold and when it is fully warmed up. 


Checking the voltage.


Another method you can use to test the coolant temperature sensor is to measure the voltage with the ignition on across the sensor terminals. The engine control unit provides a reference voltage which is usually 5 volts, it also has another wire for the sensor ground. You need to check the reference voltage and the ground first because the sensor resistance normally drops as well as the voltage as the engine warms up.


You need to check the voltage when the car is cold and when it is fully warmed up. The multimeter should show 5 volt when the sensor is disconnected. If the reading says that there is no voltage, it could mean that the circuit is open or shorted to ground.    


Checking the sensor readings by comparing it to other temperature sensors.


There are other sensors that can measure the temperature such as the intake air temperature sensor. You can check the readings of the temperature of both sensors and compare if their numbers are close. This can be done to a car that has been parked overnight since it usually has almost similar readings for both the engine coolant temperature and the intake air temperature sensors. There is a small difference since the air warms up quicker than the metal engine. After you check both of the sensors' temperature and find out that the difference of the temperature is much bigger, it could mean that one of the sensors is faulty and gives inaccurate temperature measures. 


Coolant Temperature Sensor: How to replace a coolant temperature sensor?  


Replacing a coolant temperature sensor can be done in your own garage if you are familiar with the engine parts. It is an easy process but if you are not that confident in doing the replacement yourself, better let the professionals handle it.


  • Find the sensor.


You need to find the sensor first. It is usually located towards the front of the engine. The sensor is small so you might need to use a light to find it. You can remove the engine cover so you can easily find it.  


  • Disconnect the connector cable from the terminal.


To disconnect the connector cable from the terminal, you need to do it very carefully since the plastic connector and the wiring tend to become brittle. If it breaks, you will have to replace it.  


  • Remove the old sensor.


Unscrew and carefully loosen the sensor using a deep socket and ratchet without applying too much pressure. Once it loosens, unscrew it by hand and remove it. The coolant tends to leak out when you remove it so have the new sensor ready to replace it. If needed, you can drain the coolant.


  •  Install the new sensor.


Clean the area of any dust or debris using a clean cloth. Place the new sensor in the threads and turn it by hand at first then tighten it with a torque wrench after. Follow the manufacturer’s instructions on how tight the sensor should be. 


  • Reconnect the connector cable. 


When you reconnect the cable, make sure that it’s clean. Carefully plug it into the new sensor and tighten any clips to make sure that the connection to the terminal is good and secure. Start the engine and monitor the temperature gauge as the engine warms up to make sure that the new sensor is working.


The coolant temperature sensor plays an important role in the car’s cooling system. The signals it sends and its readings can help make sure that the engine is operating at its optimum operating temperature. If you suspect that your coolant temperature sensor is failing, have it checked immediately to avoid other potential problems.  



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