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Engine Temperature Sensor: How Does It Work and How Can You Spot a Faulty Sensor?

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An engine temperature sensor is a part of a car that is not often heard of but it plays a very important role in the engine’s overall performance. Lots of essential engine operations rely on the engine temperature sensor to work efficiently such as the fuel injection timing, ignition timing, and air to fuel mixture ratio selection. 

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A lot of functions rely on the small engine temperature sensor, but what really is it and how does it work? Here is our ultimate guide that provides information about the engine temperature sensor, how it works, and more. 


Engine Temperature Sensor: How does an engine temperature sensor work?


The engine temperature sensor is one of the many types of sensors found in a vehicle and its primary job is to measure the temperature of the liquid coolant in the cooling system. It works together with the engine control unit or ECU which is known to be the brain of the engine. 


The engine temperature sensor monitors the temperature and measures it and will then send the data to the engine control unit. When the ECU receives the said data, it analyzes it, processes it and will then execute the needed action. It will adjust or regulate the air-fuel mixture ratio and the ignition timing. If the engine gets too hot, less fuel will be needed and more fuel is supplied when the engine gets too cold. 


The engine control unit can control and adjust the car’s engine performance based on the signals it receives from the sensors. It can also operate the electric radiator fans when it senses that the temperature will go beyond the engine’s optimal working temperature range. If the sensors are not detected or are now beyond the expected range, the engine control unit will turn on the Check Engine light and will put the appropropriate trouble codes into its memory. The engine temperature sensor is the one that also provides readings for the dashboard’s engine temperature gauge. 


Most engine temperature sensors are negative temperature coefficient thermistors. This means that when the temperature increases, their electrical resistance decreases. The engine temperature sensor uses this electrical resistance to get an accurate measure of the coolant’s temperature. This type of sensor is immersed in the coolant. 


Lots of cars usually have more than one engine temperature sensor. The first one or the primary engine temperature sensor is usually located near the thermostat in the cylinder head or on the thermostat housing and the second one is installed in the radiator or in another engine part.


There are some cars that use a cylinder head temperature sensor instead of an engine temperature sensor. There are also some cars that use these two sensors together. Unlike the engine temperature sensor that is immersed in the engine’s coolant and measures the temperature through it, the cylinder head temperature sensor measures the temperature of the cylinder head metal and accurately reads it even with the absence of the coolant. This is found to be effective in preventing overheating. 


Engine Temperature Sensor: How do you know if your engine coolant temperature sensor is bad?     


The engine temperature sensor can go bad over time and the only way to fix it  might only be by replacing it. A bad engine temperature sensor can cause a series of issues that can escalate quickly to a more serious problem if not fixed immediately. To help you prevent that from happening, it is important that you know the early signs of a bad engine temperature sensor. 


Here’s our list of the common symptoms of a faulty engine temperature sensor.


  • Check engine light is turned on.


 An illuminating check engine light is one of the early symptoms of a faulty engine temperature sensor. This is because when the engine control unit detects that there is a problem with the engine temperature sensor, it automatically sets off the check engine light to let you know that it needs to be inspected or fixed.    


  • Increased fuel consumption.


If you noticed that your vehicle is consuming more fuel than usual, it might be because of a bad engine temperature sensor. A faulty sensor tends to send inaccurate signals or readings to the ECU. Not knowing that the data that has been received is incorrect, the ECU will still process it and will base its next course of action from it. 


If the engine temperature sensor sends data that the engine is cold when it’s not, the ECU will assume that the data is correct and will adjust the air-fuel mixture and will use more fuel to help warm up the engine quickly. This causes poor gas mileage and a decreased performance of your engine. 

  • Electric cooling fan not working.


The engine temperature sensor monitors the temperature of the engine to make sure that it runs at its appropriate operating temperature range. When the ECU receives the data from the engine temperature sensor, it can trigger the electric cooling fan to start or shut off depending on the needs of the engine. This happens because cars usually use an engine temperature sensor in controlling the cooling fans.


An out of order engine temperature sensor can cause your electric cooling fan to not work. Sometimes, it causes it to stay on all the time, too. If this happens, you can try to disconnect its wires from the sensor and reconnect them again. If it doesn’t work, you might need to replace the faulty sensors. 


  • Overheating.


If a bad engine temperature sensor continues to send cold signals to the ECU when it’s not, it could cause your engine to overheat. Thinking that your engine is cold, the ECU will continue to help it warm up instead of helping it to cool down. This leads to an overheating engine. Car misfire and engine knocking can happen too. 


  • Thick black smoke from the exhaust.


Thick black smoke coming from your car’s exhaust is also one of the most common symptoms of a bad engine temperature sensor. This happens because when the sensors send the wrong temperature readings, the ECU proceeds to adjust the air-fuel mixture thinking it is what the engine needs. When the air-fuel mixture gets too rich, it will be difficult to burn the fuel up in the combustion chamber. It will be burned up in the exhaust pipes instead which produces the thick, black smoke. 


Engine Temperature Sensor: How do you test an engine temperature sensor?   


To make sure that your engine temperature is in good working condition, you can do some tests and inspections. Here are some of the things you need to inspect to help you diagnose a faulty engine temperature sensor. 


  • Check for severe corrosion around the terminal and inside the engine coolant temperature sensor. .
  • Check for any cracks in the sensor.
  • Check for water contaminations in the sensor connector.
  • Search for coolant leaks around the sensor. 


Keep in mind that although a visual inspection can really help, there are also some problems that occur without any visible signs of sensor defects. When a visual inspection couldn’t help you find the problem at all, you can test your engine temperature sensor by measuring its resistance and voltage readings.


To test it, you can use these following methods.

  • Using a scan tool. 


There are vehicles that offer direct access to the sensor data by using a scan tool. If you use it, the output of the temperature sensor is usually displayed in degrees fahrenheit or  in degrees celsius. When your engine is cold, the output should also be low and if your engine is warm the output should also be high at around 200 to 230 degrees fahrenheit. 


Any reading that doesn’t make sense and doesn’t match with the engine temperature, or a reading that doesn’t change means that your temperature sensor or its wiring is faulty. 

  • Using a multimeter.


Using a multimeter, you can  check the internal resistance of your engine temperature sensor. You can test it by following these simple steps.


  1. Make sure that your car is turned off to avoid shocks. Open your hood and locate the engine temperature sensor. You can just follow the upper radiator hose towards your engine. You can see it next to the thermostat. It looks like a black device that is plugged into a wire harness. 


  1. Unplug and remove the temperature sensor from your car. Do it gently so you won’t damage its wirings and any other parts. 


  1. Fasten the leads of the multimeter to the temperature sensor’s outer connectors. If you inspect the sensor, you will see three connectors similar to metal prongs. Attach the multimeter’s red lead to either the far left or the far right connector and clip the black lead on the opposite connector far side across. Make sure that the two leads don’t touch each other. 


  1. Check its resistance by taking a cold and a hot reading. For the cold reading, get a clean cup and fill it with clean water and some ice cubes. Wait until the water becomes cold and check its temperature by using a thermometer. If it reads 33 °F or 1 °C, you can proceed to the next step. If not, wait until the desired temperature is achieved. Make sure to check the water with a thermometer first.


  1. If the water is cold enough, turn on the multimeter and set it to DC. Do all this while its leads are still connected to the temperature sensor. Then submerge the temperature sensor in the cold water gently. Write down the reading. You should get a reading of around five volts.


  1. For a hot reading, put about 180 ml of clean water in a pot and wait for it to boil. If the water is boiling, pour it into a mug and submerge your temperature sensor in there and wait for a few minutes. Write down the multimeter’s reading. You should get about .25 volts. 


  1. Since not all engine temperature sensors are the same, each car has a specific temperature sensor depending on its make and model, you can search online to find out the specific hot and cold readings of your car. Compare it with the readings you’ve written down when you tested your car’s temperature sensor. If they closely match, it means that your sensor is working properly. If it’s not, you will need to replace your engine temperature sensor.  


Engine Temperature Sensor: How long does an engine coolant temperature sensor last?


The car’s engine temperature sensor gets bad eventually and a replacement will be the only way to fix it. It has been said that the engine temperature sensor needs to be replaced after your car hits 100,000 miles. But a poorly maintained engine cooling system tends to cause damage on your engine temperature sensor even way before your car hits that mark. 


Once your engine temperature sensor is failing, shorted, or gives incorrect readings, you should make sure to have it replaced immediately so your engine management system can function properly.  


It is also recommended that when you replace or rebuild your engine, you should also install a new engine temperature sensor. Doing so can help prevent a lot of potential problems in the future. 


Another thing, when your car experiences a severe overheating, it is always a great habit to have its engine temperature sensor and thermostat replaced. Extremely high engine temperatures can damage the sensor and thermostat’s components that can cause them to act out or fail prematurely. 


Keep in mind that when you replace your engine temperature sensor, you will have to drain some of the coolant from the cooling system just enough so the engine coolant level is below the temperature sensor. When you do this, check the condition of your coolant as well and replace it if needed. 


The engine temperature sensor is one of the most important parts of the whole engine management system. The engine control unit relies on its readings and it affects the engine’s performance as well. That is why you need to pay attention to its early warnings and signs when it’s about to go bad to prevent an even bigger problem from happening. You need to have it checked by a professional immediately. 

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