Bad Coolant Temperature Sensor Symptoms, Causes & How To Test It

Table of Contents

1 What is an Engine Coolant Temperature Sensor?
2 Symptoms Of a Bad Coolant Temperature Sensor
3 Causes of Bad Coolant Temperature Sensor
4 Coolant Temperature Sensor Location
5 Engine Coolant Temperature Sensor Replacement Cost
6 How to Test an Engine Coolant Temperature Sensor?
7 How to Replace a Coolant Temperature Sensor?
8 FAQ Section

The engine coolant temperature sensor or ECT sensor plays a crucial role in ensuring the efficient functioning of your vehicle. Its primary function is to measure the coolant temperature and send this data to the engine control module (ECM). Based on this data, the ECM adjusts the air-fuel mixture accordingly. When the engine coolant temperature sensor goes bad, it can cause various issues with your car’s performance. This article explains the bad coolant temperature sensor symptoms, causes, and replacement cost.

What is an Engine Coolant Temperature Sensor?

A coolant temperature sensor, also known as an engine coolant temperature (ECT) sensor, is a part of your vehicle that monitors the temperature of the engine coolant and sends this data to the powertrain control module (PCM) or engine control module (ECM).

The coolant temperature sensor is usually located in the engine’s cylinder head or near the thermostat. It uses a thermistor, which is a resistor. The resistance of the thermistor varies according to the temperature.

When the coolant temperature varies, the thermistor resistance also changes. The coolant sensor measures this resistance and converts it into an electrical signal that is sent to the ECM.

Symptoms Of a Bad Coolant Temperature Sensor

When the coolant temperature sensor goes bad, it produces one or more of the below-given symptoms:

1) Check Engine Light

An illuminated check engine light is one of the most common signs of a bad engine coolant temperature sensor.

When your vehicle’s ECM monitors an issue with the coolant temperature sensor, it activates the check engine light on the car’s dashboard. This light serves as a signal for the need to have your vehicle inspected, as the computer system detects abnormalities in the sensor’s circuit.

To diagnose the problem, it is advisable to connect an OBD-II scanner to retrieve the trouble codes associated with the check engine light.

Read More: How To Reset Check Engine Light?

2) Faulty Water Pump

A faulty engine coolant temperature sensor may potentially lead to a bad water pump. If the vehicle’s water pump fails prematurely, it could be an indication of an issue with the ECT sensor.

This situation arises when the defective sensor provides readings that are higher than the actual coolant temperature. As a result, the ECM of the vehicle takes corrective actions to address the perceived overheating.

In such conditions, the ECM increases both the fuel injection rate and the speed of the water pump to circulate more coolant and cool down the engine as quickly as possible.

However, this places undue strain on the water pump, causing it to wear out prematurely before reaching its expected lifespan.

3) Poor Fuel Economy

A faulty coolant temperature sensor can contribute to poor fuel economy as one of its symptoms. When the ECT sensor sends the wrong data to the ECM, it may result in a wrong air-fuel mixture.

For instance, a bad sensor may indicate that your engine is cold even if it is hot, causing the ECM to deliver more fuel for quick engine warm-up. This, in turn, leads to higher fuel consumption and reduced engine performance.

Read More: Poor Gas Mileage Causes

4) Poor Engine Performance

A defective ECT sensor may badly affect the overall performance of your car. It can provide inaccurate readings, signaling low coolant temperature or falsely indicating high coolant temperature when it is not the case.

As the engine control module (ECM) gets information from the coolant sensor indicating high coolant temperature, it interprets it as an overheating engine. As a result, the ECM adjusts the fuel injection ratio to properly cool the engine promptly.

Running on a leaner air-fuel mixture due to reduced fuel input can help the engine maintain optimum performance. However, this disturbance in engine operational settings can make the vehicle feel sluggish during acceleration.

5) Overheated Engine

In your vehicle, the cooling fan, located behind the radiator grille, plays a crucial role in properly cooling the coolant. It operates based on electrical signals received from your vehicle’s ECM or PCM.

If the coolant temperature sensor provides inaccurate data, the fan may fail to activate, resulting in engine overheating. While some vehicle models use a separate coolant temperature sensor specifically for the fan, but most models utilize the same sensor for both functions.

6) Cooling Fan is Not Working

A bad coolant temperature sensor may provide inaccurate readings, indicating a coolant temperature that is either higher or lower than the real temperature.

By using this erroneous information, the ECM adjusts the speed and activation patterns of the cooling fans.

Consequently, you may observe instances where the cooling fan operates when it is not required or fails to activate when it is necessary, indicating a faulty coolant temperature sensor.

7) Hard Starting

The initial startup of a vehicle is particularly crucial in terms of fuel injection. If the air/fuel mixture is imbalanced due to a bad coolant temperature sensor, you may experience difficulties or even an inability to start your vehicle.

Read More: Why Your Car Won’t Start?

8) Black Smoke from Tailpipe

A defective ECT sensor may disrupt the air-fuel ratio in your engine cylinder, affecting the combustion process. An ideal fuel and air supply is necessary for the complete combustion process. If there is an issue in the delivery of the fuel, the combustion will not complete.

A wrong signal from the coolant temperature sensor can result in either insufficient or excessive amounts of fuel being delivered to the engine.

In case of excessive amounts of fuel supply, the excess fuel remains unburned and accumulates in the combustor. This unburned fuel then exits through the exhaust system, causing the emission of black smoke from the muffler.

Regardless of whether there is too little or too much fuel being injected, it is important to address issues related to the ECT sensor.

9) Transmission Shifting Issues

The coolant temperature sensor readings are utilized by the Transmission Control Module to prevent your vehicle from shifting into overdrive when the engine is cold. If the sensor provides wrong data, it may result in transmission issues and a decrease in engine efficiency.

10) Poor Idling

Poor idling is one of the major symptoms of a bad coolant temperature sensor. A bad ECT sensor can lead to adjustments in the fuel mixture, causing the engine to vibrate at low speeds. This can also result in power losses and unusual behavior from the engine.

Idle conditions are particularly sensitive to an incorrect air-fuel mixture, and this is when you may observe indications of a malfunctioning ECT sensor.

Causes of Bad Coolant Temperature Sensor

Your vehicle’s coolant temperature sensor goes bad due to one or more of the following causes:

Faulty Connections to the Sensor
Overheating or Thermal Stress
Insufficient Coolant and Air Pockets
Corrosion on Sensor Terminals
Faulty Thermostat

1) Faulty Connections to the Sensor

A damaged or faulty electrical connection between the ECM and the engine coolant temperature sensor is one of the major causes of a faulty coolant temperature sensor. These connections may be damaged when they come into contact with rotating parts of the vehicle.

2) Overheating or Thermal Stress

Constant exposure to high temperatures or extreme temperature variations may reduce the service life of the sensor. Overheating may lead the sensor to become permanently damaged or lose its calibration.

3) Insufficient Coolant and Air Pockets

The low coolant levels and the presence of air pockets within the cooling system may have a detrimental impact on the accuracy of the ECT sensor reading.

Insufficient coolant may lead to the engine overheating, causing poor engine performance and the illumination of the check engine light.

4) Corrosion on Sensor Terminals

Corrosion on the ECT sensor terminals can occur because of water leakage. This corrosion may lead to the sensor malfunction, resulting in the ECM registering a trouble error code.

5) Faulty Thermostat

A bad thermostat may disturb the flow of coolant, leading to wrong temperature readings or fluctuations that may affect the performance of the coolant temperature sensor.

Coolant Temperature Sensor Location

The specific placement of the coolant temperature sensor can vary depending on the vehicle’s design and the brand or manufacturer. Different vehicles may employ various configurations for sensor location.

The coolant temperature sensor is typically positioned on the cylinder head or engine block. It is commonly found on a plastic hose connected to the inlet of the coolant system.

In certain cases, there may be multiple temperature sensors present in a vehicle. These sensors serve different functions, such as communicating with the ECM, controlling the cooling fan, or signaling the dashboard.

When two sensors are present, typically, the one responsible for relaying signals to the ECM is considered the primary coolant temperature sensor.

Engine Coolant Temperature Sensor Replacement Cost

The replacement cost of the coolant sensor depends on the vehicle model and labor cost. The average replacement cost of the engine coolant temperature sensor is from $40 to $260. The coolant temperature sensor itself costs from $20 to $120, and the labor costs between $20 and $140.

How to Test an Engine Coolant Temperature Sensor?

Follow the below-given steps to test a coolant temperature sensor:

Ensure that the engine is cool before proceeding with the test to avoid any potential burns.
Use a scan tool such as the OBD-II scanner to retrieve the stored error codes related to the coolant temperature sensor. Properly read the freeze data to observe the temperature reading from the sensor. If the reading is significantly outside the expected range, proceed to the next steps.
Find the coolant temperature sensor. It is typically positioned on the cylinder head or engine block. Consult your vehicle’s service manual for its precise location.
Disconnect the connector plugs.
If the sensor has two pins, you can measure the ohm value between these two pins using a multimeter.
Connect the probes of the multimeter to the terminals of the coolant temperature sensor and read the reading.
Consult your vehicle’s repair manual to find the correct ohm value corresponding to the measured temperature.
If the measured value deviates from the expected range, it indicates a faulty sensor, and it should be replaced.
If the resistance reading is within the expected range, it suggests that the coolant temperature sensor is functioning correctly.
Inspect the wirings and connectors between the coolant temperature sensor and the ECM for any signs of corrosion, damage, or loose connections.

How to Replace a Coolant Temperature Sensor?

Follow the below-given steps to replace the engine coolant temperature sensor on your car:

Park your vehicle in a level area.
Allow your vehicle’s engine to cool down for approximately 10 to 16 minutes to prevent yourself from a burn injury.
Find the coolant temperature sensor. It is typically positioned on the cylinder head or engine block. Consult the vehicle’s service manual for precise location information.
Place a drain pan beneath the vehicle to catch any coolant that may leak out during the sensor removal process.
Gently disconnect the electrical wires from the sensor terminal, ensuring not to damage the connector or wires.
Unscrew and remove the old coolant sensor from its position.
Take the new sensor and securely screw it into its original place in a clockwise direction. Use a torque wrench to tighten the sensor according to the service manual’s recommendations.
Connect the electrical connectors, wires, and other parts back to their original place and ensure a secure connection.
Turn on the engine and allow it to warm up.
Monitor the temperature gauge on the dashboard to ensure that the sensor accurately reflects the changing engine temperature.
Verify the functionality of the new sensor by observing the temperature gauge and ensuring it corresponds to the engine’s temperature changes.

FAQ Section

How long does it take to replace a coolant temperature sensor?

The time required to replace a coolant sensor can vary depending on the specific vehicle model and your skills. Generally, it takes approximately 25 to 65 minutes to replace a coolant temperature sensor.

What is a cylinder head temperature sensor?

The cylinder head temperature sensor serves a different purpose than the coolant sensor. It directly calculates the temperature of the cylinder head and is often used in conjunction with the coolant temperature sensor in some vehicles. It may also be utilized as a separate sensor in air-cooled engines.

Can a bad coolant temperature sensor prevent a car from starting?

Indeed, a faulty coolant temperature sensor may prevent your car engine from starting. For instance, if the sensor erroneously indicates a warm temperature while it’s actually cold outside, the engine control module (ECM) may not inject enough fuel to facilitate engine startup.

Do you have to drain the coolant to replace the temperature sensor?

In most vehicles, replacing the coolant temperature sensor requires draining the coolant. However, in certain situations where the sensor is positioned high on the engine, it may be possible to rapidly switch the sensor without fully emptying the coolant system. Always ensure that the coolant is cold before working on the engine.

Is there a fuse for the coolant temperature sensor?

There is typically no dedicated fuse specifically for the coolant temperature sensor itself. However, your vehicle may have a fuse for the engine control module, which is responsible for sending signals to the ECT sensor.

Can a bad coolant temperature sensor cause overheating?

Yes, a malfunctioning coolant temperature sensor may lead to engine overheating, posing a risk of significant engine damage. Therefore, you must have knowledge about the associated symptoms and address the issue promptly by replacing the sensor if it is suspected to be faulty.

Does the coolant temp sensor control the fan?

The coolant temperature sensor of your vehicle doesn’t directly control the cooling fan, but the input of this sensor plays a vital role in determining when the fan should be activated or deactivated. The engine control module (ECM) of your vehicle utilizes the data of this sensor to regulate the fan operation.

Written By

Engineer Rahela

Rahela is the owner and main author of Mechanicalspecial. She is an automotive expert writer. She have over four years of experience in the automotive writing niche and have completed over 700 pieces of content from 40 projects. She have good knowledge and skill in vehicle repairing, all mechanical work, car upgrades and maintenance.