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5 Symptoms of a Bad Coolant Temperature Sensor

By September 15, 2021Maintenance
Coolant Temperature Sensor - Gearstar

A coolant temperature sensor remains one of the most vital engine management sensors. Learn why and symptoms of a bad sensor.

The coolant temperature switch – otherwise known as the coolant temperature sensor – is primarily responsible for monitoring the engine coolant’s temperature. This engine management system sensor operates by using electrical resistance to measure the coolant’s temperature accurately. 

The signal generated is automatically transmitted to the computer so that necessary changes can be made. This includes changing the engine’s timing as well as the fuel calculations for premium performance. Most automobile engines require more fuel when they are cold but require less when fully warmed up.

If the computer detects that the engine’s temperature is too high, it will automatically scale back engine performance settings. This occurs to prevent overheating, which can damage the engine severely.

As you can see, temperature plays such a crucial role in the performance calculations of your engine. And this implies that if the coolant temperature sensor malfunctions, it may translate very quickly to engine performance hiccups.

This post discusses some of the common symptoms of a bad coolant temperature sensor. These symptoms should alert you of potential problems with your automobile and push you to have it checked out thoroughly.

Symptoms of a Bad Coolant Temperature Sensor

Overheating Engine 

If you notice that your engine overheats, the coolant temperature sensor could be the problem. At times, the coolant temperature switch may fail such that it starts transmitting hot signals permanently. 

This causes the computer to erroneously counterbalance the signal, thereby causing the engine to misfire or overheat. 

Poor Fuel Economy

Poor fuel economy is another symptom that indicates your coolant temperature sensor is malfunctioning. 

If the coolant temperature sensor malfunctions, it can transmit a false signal to the computer. And the response of the computer may unbalance the timing and fuel calculations. 

It is also common for a coolant temperature sensor to fail without warning and send permanently cold signals to the computer. The computer mistakenly assumes the engine is cold, even when it is not. 

This will cause the automobile to use up more fuel than is necessary, thereby reducing fuel economy. And this will ultimately hamper engine performance.

‘Check Engine Lights Turns On

If the ‘Check Engine’ light suddenly comes on, this could imply a hitch with the coolant temperature sensor. If the computer detects an unsavory development with the sensor’s circuit or signal, it may set off the ‘Check Engine’ light. And the light will not go off until a specialist readily addresses the issue. 

Black Smoke Emitted from the Engine

Few drivers hardly notice this crucial coolant temperature sensor symptom. But if your vehicle’s exhaust starts emitting black smoke, the coolant temperature sensor could be the culprit.

The coolant temperature sensor can fail and transmit cold signals to the computer. This action confuses the computer and stimulates it to enrich the fuel mixture unnecessarily. This can cause the fuel mixture to be vibrant such that during combustion, the fuel can’t be adequately consumed within the chamber.

This leads to the burning up of your vehicle’s exhaust pipe, thereby causing black smoke. This can be so severe that driving the vehicle in such a condition may result in environmental or excessive air pollution.

Poor Idling

A faulty sensor can cause the fuel mixture to adjust. This causes the engine to shake or vibrate when the vehicle is at low speed and can even bring about strange behaviors, power losses, etc.

Engines are highly sensitive, especially to inappropriate air-fuel mixtures at idle. This is a pointer that something is wrong with your coolant temperature sensor and must be checked out by a certified professional.

Defective Electrical Cooling Fans

Some vehicles’ coolant temperature sensors control the electric cooling fans. Most automobiles come with two different temperature sensors for the fans, engine management and dashboard gauge.

However, if your vehicle only has a single coolant temperature sensor and it becomes faulty, your electrical fans may not function at all.

Where is the Coolant Temperature Sensor Located?

Different car manufacturers and brands have distinct ways of installing the coolant temperature sensor. Its exact location also depends on its overall design.

However, the coolant temperature sensor is often located right on the cylinder head or engine block. You will find it installed conveniently on a plastic hose on the coolant’s inlet.

Some automobiles come with more than one coolant temperature sensor. This is because sometimes, these different switches can be used to send signals to the cooling fan control, dashboard, and control unit of the engine system.

If your vehicle comes with two sensors, the one that transmits signals to the control unit is often known as the 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.


The coolant temperature switch remains one of the most vital engine management sensors. This is because it plays a crucial role in numerous calculations that ultimately affect your engine’s performance.

This is why you shouldn’t waste any time if you suspect that the coolant temperature sensor is malfunctioning. Get a professional or experienced technician to inspect your vehicle as soon as possible.

The technician should be able to analyze or diagnose your automobile and determine if there will be a need for the replacement of the coolant temperature sensor or not.


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