Symptoms of a Bad Coolant Temp Sensor

The coolant temp sensor is one of the car engine management systems that monitor the temp of the radiator coolant. The coolant temp sensor functions perfectly by monitoring the coolant temp with the aid of an electrical circuit. The sensor sends a signal to the car computer to make changes for gas calculation and engine timing for peak performance.

As the vehicle engine requires less gas when warmed and more gas when cold, the car computer will adjust the engine performance setting if it receives a signal that coolant temp is above the normal operating temperature to save the engine from catastrophic damages due to the engine overheating.

Because the coolant temperature sensor plays a critical role in the engine timing and calculation for optimal performance, a lousy coolant temp sensor can easily affect your engine’s performance. Hence, you should always watch out for symptoms of a bad coolant temp sensor. In this article, we’ll discuss at length how to tell if your coolant temp sensor is bad, the signs and symptoms of a defective coolant temp sensor, and how to fix the problem.

Symptoms of a bad coolant temp sensor

Bad Coolant Temp Sensor

Every mechanical component in your vehicle emits symptoms to notify the driver when they fail, and the coolant temperature sensor is no exception. So, before a coolant temp sensor failure escalates to more significant auto repair for your transmission or engine, it will give the following signs;

Irregular temperature readings:

If you notice temperature fluctuations in your temperature gauge, or it stays lower or higher than expected when the engine is running, it could be an indicator of a failing or lousy coolant temp sensor. Especially if the engine doesn’t warm as it supposes to, you have to contact your mechanic to diagnose and track down the problem in your cooling system.

While the coolant temperature might be cooler than the ambient air, the coolant temperature can send inaccurate readings to the car computer. However, keep in mind that aside from faulty coolant temp sensor, there are other host causes of irregular or fluctuating temperature; such as cylinder head leak, low coolant level, failing radiator fan, and bad pressure cap.

Black smoke from the exhaust:

A common sign of a defective coolant temp sensor is black smoke from the exhaust tailpipe. A failing coolant temp sensor will send a cold signal to the car computer, thereby misleading and causing the computer to unnecessarily enrich the air/fuel mixture ratio. If the air-fuel mixture becomes rich to a state where the gas cannot be appropriately burned during the combustion process, it will cause black smoke from the exhaust tailpipe. In severe cases, the smoke may be big enough that you don’t want to drive the vehicle with it.

Engine overheating:

while it is pretty clear that a failing or faulty temp sensor will cause a cold reading, in some cases, the coolant temp sensor can malfunction and send a hot signal when the engine is running at an average operating temperature. In such a case, your car computer will think the vehicle is running at a high temperature when it’s not. As a result, it’ll trigger a dashboard warning light, and your car will operate sluggishly.

Poor fuel economy:

One of the most common signs of a bad coolant temp sensor is poor fuel economy. A bad coolant temp sensor will send a false reading to the car computer and disrupt accurate gas and timing calculations. Commonly, a failed temp sensor might permanently send a cold signal to the computer, making it think the vehicle is always cold even when it’s running hot. As a result, it reduces gas mileage and disrupts engine peak performance.

Rough Idling:

Can a bad coolant temp sensor cause rough idle? Yes, due to the false signal sent to the car computer, it will adjust the air-fuel mixture. The supplied mixture will not be enough and result in vibrations and shaking at idle or low-speed maneuvers and lead to other strange behavior and loss of engine power.

Check engine light:

An illuminating check engine light signifies a potential problem in the internal engine components or a faulty electrical system. Several factors can trigger the engine warning light. To quickly figure while the light comes on, you need a scan tool to diagnose the car. The diagnostic tool will read and translate the fault codes in alphanumeric order. Each represents a specific problem. Related coolant temp fault codes are;

  • P0115: this means a random issue with the coolant temp circuit.
  • P0116: this means a problem with the coolant temp circuit range.
  • P0117:  this means a low input in the coolant temp circuit.
  • P0118: this means a high input in the coolant temp circuit.
  • P0128:  this indicates coolant temp is below the thermostat average operating temp.

How to fix coolant temp sensor problems

You might think the best solution to this problem is to change the sensor. Well, sometimes, you don’t have to replace the sensor. Therefore, it is imperative that you know how to test coolant temp sensors using a diagnostic machine, an oscilloscope, or a digital multimeter. For instance, the coolant temp sensor will send false readings for a low coolant level. If this is the case, you will fix the problem by topping your coolant to the appropriate gauge.

how to test coolant temp sensor

Working under the hood can be messy, especially when working on an area you haven’t cleaned for long.  So, you need a mechanic glove, eye protection, and long sleeve shirt. Organizing and keeping your mechanic tools close by will save you time in completing the work. Replacing a temp sensor is an easy task and can be done by anyone familiar with the ‘underhood’ components. Follow the below steps to replace a defective temp sensor.

Locate the temp sensor: The sensor is typically located near the radiator or on the thermostat compartment. In some vehicles, the temp sensor is located at the back of the engine, so you may need a touch light to locate it. The thermostat housing is connected to the upper radiator hose; follow the upper hose to locate the thermostat housing and find the CTS on that area.

Remove the electrical connectors: The temp sensor is connected to the car computer via an electrical connector. You need to carefully unplug the connector as it can be brittle and break. Always inspect electrical connectors for corrosion and clean them if you find any.

Unfasten and remove the sensor: Coolant temp sensors are typically installed like a spark plug, so you need a long socket and ratchet handle to unscrew it. If the sensor proves stubborn to break free, do not apply much pressure. Rather a squirt of break-free solvent will help slack it easily. Remember to remove the radiator cover before removing the sensor. Once the CTS is loose, unscrew it by hand. The antifreeze will leak from this spot, so ensure you refill it after all.

Compare both sensors and install the new one: place both sensors together and compare them. This is usually necessary if you purchase an aftermarket part. Head over to the CTS housing and clean the debris from the old sensor with a rag. Insert the new CTS into the housing and thread it by hand. Once you have incorporated the sensor and ensure it didn’t miss any thread, tighten it with a torque wrench to the manufacturer’s specified range.

Reinstall the electrical connectors: Once you have tightened the sensor, the next thing is to reinstate the electrical connector. Ensure there is no corrosion on the electrical connectors and plug it back to the CTS. Plug back any fastener on the sensor and make sure it’s well fitted. Refill the coolant and put back the pressure cap on the radiator. Start the engine and wait to see if the temp gauge is reading correctly.

Test the Vehicle: Test run the car to see if your efforts have finally paid off. Once you’re back from the test drive, recheck the coolant level and see if it has dropped and refill it. If everything is nice and smooth, give yourself a pat on the back.


Q: How long does a coolant temperature sensor last?

Defective coolant temperature sensors are common. If you fail to carry out regular and scheduled maintenance specified by your manufacturer, it can corrode the CTS endpoint that touches the coolant. Coolant temp sensors can last as long as 100,000 miles. However, if your engine cooling system lacks proper maintenance, the sensor can fail early.

Q: What does a coolant temperature sensor do?

The coolant temperature sensor monitors and measures the temperature of the engine coolant in the cooling system and tells the ECU how much heat the radiator expelled out of the coolant. The coolant temp sensor works hand in hand with the ECU, continually measuring and monitoring the coolant temp to ensure the vehicle engine works at average operating temp for peak performance.

Q: Does the temperature sensor control the fan?

The temp sensor works by monitoring and measuring the coolant temperature. The sensor then sends the temperature readings to the ECU, and as the ECU receives the reading, it may cause the radiator fan to either switch on or switch off.

Q: Does the coolant temperature sensor affect AC?

In a proper system operation, the coolant temp shouldn’t affect the air-conditioning system. In any case, since the radiator fan helps the condenser fan in cooling the AC condenser, if the ECU shut off the radiator fan as a result of false readings from the CTS, the AC condenser will lack enough air to cool it down, thereby affecting the optimum functionality of the entire AC system.

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.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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