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Bad Coolant Temperature Sensor: Causes + What to Do About it

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Is your engine overheating, even when it’s not too hot outside, and you haven’t been driving all day long either?

Is the check engine warning sign also lit on your dashboard?

Well, it could be the Engine Coolant Temperature (ECT) sensor that’s causing these issues.

In this article, we’ll cover what a coolant temperature sensor is, the main signs and causes of its failure, and how to diagnose and fix it. We’ll also answer a few FAQs related to the ECT sensor.

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Let’s get to it.

What Is an Engine Coolant Temperature Sensor?

The coolant temperature sensor measures the temperature of the engine coolant through a resistor circuit. It conveys this reading to the Engine Control Module (ECM), also referred to as the Engine Control Unit (ECU), which acts as the “brain” of your car.

How is it done?

The ECM supplies the sensor with a constant reference voltage of 5V which the sensor converts into a suitable voltage signal according to the coolant temperature. Your car’s coolant temperature sensor usually has a Negative Temperature Coefficient (NTC), which means its resistance decreases with a rise in coolant temperature, resulting in a reduced voltage output signal.

The ECM detects a change in the voltage signal from the sensor and uses it to control:

Your car’s cooling system may have multiple coolant temperature sensors, with the primary sensor located on the engine block near the thermostat housing or the radiator.

Now let’s discuss the eight signs of a faulty sensor that can take a toll on your car’s engine performance.

8 Signs of a Faulty Coolant Temperature Sensor

Here are the key symptoms that accompany coolant temperature sensor failure:

1. Engine Overheating

A malfunctioning coolant temperature sensor may send an incorrect “cold” signal to the ECU, tricking it into believing the engine isn’t hot yet. The ECU will then adjust the fuel injection, ignition timing, and variable valve timing, further raising the engine’s temperature and causing it to overheat

2. Illuminated Check Engine Light

A faulty sensor can result in the ECU activating the Check Engine Light on your car’s dashboard. It will also register an error code that can be read using an OBD-2 scan tool.

3. Black Smoke from Exhaust

Your car’s coolant temperature sensor helps its ECU decide the fuel-air mixture ratio for combustion. A bad sensor may result in a rich fuel-air mixture, leading to black smoke from the exhaust pipe and poor fuel economy.

4. Difficulty Starting the Car

When you cold-start your car, the ECU calculates the required fuel-air mixture ratio by considering the coolant and ambient temperatures. A bad coolant temperature sensor may result in a lean fuel-air mixture, causing a difficult cold start.

5. Fluctuating Temperature Gauge

The engine temperature gauge on your car’s dashboard gets its input from the coolant temperature sensor. A faulty sensor can cause it to fluctuate erratically as you drive.

6. Rough Idling

A malfunctioning ECT sensor may cause the Engine Control Unit to inject a fluctuating amount of fuel into the engine, resulting in rough idling.

7. Radiator Fan Problems 

Your car’s Engine Control Unit may turn on the radiator fan even when the engine isn’t hot enough due to a bad coolant sensor. The opposite of this can also happen, resulting in the engine heating up beyond its operating temperature and getting damaged.

8. Transmission Shifting Problems

The Transmission Control Module (TCM) uses the coolant temperature sensor reading to prevent the car from shifting into overdrive while the engine is cold. Incorrect information from the sensor can lead to transmission problems and reduced engine performance.

Now, let’s have a look at the possible reasons behind a faulty coolant temperature sensor.

3 Common Reasons a Coolant Temperature Sensor Goes Bad

Listed below are the leading causes of coolant temperature sensor failure:

1. Faulty Connections to the Sensor

The electrical connections to the coolant temperature sensor from the Engine Control Module (ECM) may get physically damaged from contact with the car’s moving parts, such as the transmission system. This can cause the ECM to generate a coolant sensor error code.

2. Corrosion on Sensor Terminals

The coolant temperature sensor terminals may get corroded due to water seepage. This causes the sensor to malfunction and the Engine Control Unit to register an error code.

3. Low Coolant Level and Air Pockets

Insufficient engine coolant and air pockets in your car’s cooling system can negatively affect the reading taken by the sensor. A low coolant level will also cause your car to overheat, resulting in reduced engine performance and an illuminated Check Engine Light.

Having understood what can cause coolant temperature sensor problems, let’s find a way to diagnose them.

How to Diagnose a Coolant Temperature Sensor Issue?

A mechanic will check your car’s coolant level before going ahead with a thorough test using a voltmeter. They’ll also check the error codes, if there are any, using an OBD scan tool.

Here are the steps they’ll follow next:  

  1. Connect the voltmeter’s positive lead to the sensor’s signal terminal and negative to the chassis ground.
  1. Cold start the engine and check the readings. The reading should be between 3V – 4V, depending on the engine’s temperature.
  1. Let the engine get warmed up to its operating temperature. The voltage reading should drop (for an NTC sensor) to 1.2V – 0.5V.
  1. If the voltmeter reads 5V, it implies an open circuit. They’ll check:
    • Signal terminal connection
    • Sensor’s ground contact
  1. If it reads 0V, this indicates a short circuit or no power supply to the sensor.
    • Check the connecting wires from the ECM to the sensor
    • Check the power supply and ground connection for the ECM
  1. If the voltmeter still reads 0V, the ECM can be faulty.

If the voltmeter’s readings are beyond the range prescribed above for a cold and hot engine and there are no connection problems, you’ll have to get the sensor replaced.

Let’s get to how that’s done.

How to Replace an Engine Coolant Temperature Sensor?

Here are the steps a mechanic will take to replace the sensor on your car:

  1. Let the engine cool down for about 15 minutes to avoid any burn injuries.
  1. Locate the coolant temp sensor on the engine block (near the thermostat housing).
  1. Place a draining pan under the car. Coolant may leak out after the sensor is removed.
  1. Carefully detach the wiring connector from the sensor terminal.
  1. Unscrew the old sensor.
  1. Screw the new sensor in the clockwise direction. Tighten it using a torque wrench in the clockwise direction as per the manufacturer’s specifications.
  1. Reconnect the connector back to the sensor. 
  1. Start the engine and let it get warmed up.
  1. Check the new sensor by monitoring the dashboard temperature gauge to ensure that it reflects the change in engine temperature.

Now we’ll get to the questions that you may have about the sensor in focus.

3 FAQs About Coolant Temperature Sensor 

Here are the answers to a few common questions about the coolant temperature sensor:

1. How Much Does it Cost to Replace a Coolant Temperature Sensor?

A coolant temp sensor replacement can range from $70 – $480. The cost depends on the model of your vehicle and the labor charges.

The part costs between $20 – $80, while the labor charges can vary from $50 – $400.

2. What Are the Various Types of Coolant Temperature Sensors?

There are three common types of engine coolant temperature sensors:

3. What Is a Cylinder Head Temperature Sensor?

The cylinder head temperature sensor directly measures the temperature of the engine’s cylinder head and is not immersed in the coolant. It’s used along with the coolant temperature sensor in some vehicles or as a standalone sensor on an air-cooled engine.

Final Thoughts

A bad coolant temperature sensor can have a detrimental effect on your vehicle through engine overheating. It’s best to know its symptoms and causes so that you can avoid paying hefty amounts for engine repairs.

If you suspect that your car has a malfunctioning coolant temperature sensor, contact AutoNation Mobile Service right away!

We’re a mobile auto repair solution available 7 days a week that offers easy booking, competitive pricing, high-quality parts, and expert mechanics who’ll resolve your car issues straight from your driveway!