Is your engine overheating, even when it’s not too hot outside and you haven’t been driving all day? Is the check engine warning sign lit?
The Engine Coolant Temperature (ECT) sensor could be responsible. In this article we will discuss what a coolant temperature sensor is, its main signs and causes of failure, and how to diagnose and fix it. We will answer a few FAQs about the ECT sensor as well.
Let’s get started.
What Is an Engine Coolant Temperature Sensor?
The coolant temperature sensor is basically a resistor circuit that measures the temperature of the engine coolant and sends it to the Engine Control Module (ECM), also called Engine Control Unit (ECU), the “brain” of your car.
How does this work?
The ECM gives a 5V constant reference voltage to the sensor and the sensor converts the 5V into the right voltage signal that corresponds to the coolant temperature.
In your car, the coolant temperature sensor will produce a Negative Temperature Coefficient (NTC), meaning as coolant temperature gets higher, the amount of resistance gets less and therefore the output signal has less voltage.
The ECM sees that the voltage signal from the sensor has changed and uses it to control.
- Fuel injection and mixing
- Ignition timing
- Variable valve timing
- Transmission shifting
- Radiator fan
- Temperature gauge on the dashboard
Your car’s cooling system could have a number of coolant temperature sensors; the main one will be located on the engine block somewhere near the thermostat housing or radiator.
There are eight signs of a failing sensor, and it can make a difference to how your car’s engine performs.
8 Symptoms of a Faulty Coolant Temperature Sensor
Here are the 8 most common signs that your coolant temperature sensor is faulty:
#1. Engine Overheating.
When a coolant temperature sensor has a fault, it can deliver a “cold” reading to the ECU. The ECU will stop calculating fuel injection, ignition timing, and variable valve timing, as it will think the engine is only at ambient temperature. This can allow the engine to overheat.
#2. Check Engine Light Illuminated.
A failing sensor can cause the ECU to enter a fault mode causing the Check Engine Light to illuminate. It will also set a trouble code in the ECU that can be accessed with an OBD-2 scan tool.
#3. Black Smoke from the Exhaust.
The ECU uses coolant temperature sensors to help calculate the fuel-air ratio to the combustion chamber. A faulty sensor can trick the ECU into providing a rich fuel-air ratio that would allow black smoke to be produced at the exhaust pipe while lowering fuel economy.
#4. Starting Problems.
When cold-starting the car, the ECU must obtain the fuel-air ratio from the coolant and the outside/ambient temperature. A failing coolant temperature sensor can result in a lower fuel-air ratio causing hard starting problems to occur.
#5. Temperature Gauge Fluctuates.
The coolant temperature gauge in your vehicle receives information from the coolant temperature sensor. A faulty sensor can cause the temperature gauge to spike and fluctuate while driving.
#6. Rough Idle.
When the ECT sensor fails, the varying signal delivers the position of the ECT that makes the fuel volume injected vary, which can produce a rough engine idle.
#7. Radiator Fan Operation.
When a faulty coolant temperature sensor sends the Engine Control Unit a “cold” signal, it is possible for the ECU to turn on the radiator fan even though the engine is not hot enough.
A faulty sensor can also prevent the ECU from turning on the radiator fan while allowing the engine to heat up past operating temperature, causing damage.
#8. Transmission Shift Issues.
The Transmission Control Module (TCM) receives inputs from the coolant temperature sensor to ensure the vehicle does not shift to overdrive while the engine is still cold.
When the coolant temperature sensor is faulty, it can affect the operation of the TCM and cause poor engine performance.
Next, let’s now look at the ways coolant temperature sensors can fail.
3 Common Causes for Coolant Temperature Sensor Failure
Here are the primary causes of coolant temperature sensor failure:
#1. Damaged Connections to the Sensor.
The electrical connections going to the coolant temperature sensor from the Engine Control Module (ECM) may get damaged from contact with the moving parts of the vehicle, such as the transmission. This can lead to the ECM creating a coolant sensor fault code.
#2. Corrosion on The Sensor Terminals.
Due to water seepage, the coolant temperature sensor terminals may get corrosion, which can ruin the sensor and generate an error code from the Engine Control Unit.
#3. Low Coolant Levels and Air Pockets.
Low levels of engine coolant and/or air pockets within your car’s cooling system can impact the accuracy of the reading from the sensor. Low levels of engine coolant will also cause overheating, resulting in diminished engine performance and an illuminated Check Engine Light.
Now that we understand what could cause problems with the coolant temperature sensor, let’s look at ways to diagnose them.
How Can You Find Your Coolant Temperature Signal Problem?
The mechanic will first start with a check of the car coolant level before starting a real test with a voltmeter and an OBD scan tool to see if there are any error codes.
- The mechanic will connect the voltmeter, positive lead to the signal terminal of the sensor, negative lead to chassis ground.
- Each engine will read a different value depending on the engine temperature, but the initial output after a cold start will be approximately 3V – 4V.
- Once the engine warms up to operational temperature, the voltage from the NTC sensor should fall to approx. 1.2V – 0.5V.
- If the voltmeter reading is 5V, this indicates an open circuit and they will check the: Signal terminal connection and Sensor ground connection.
- If the reading is 0V, this can indicate a short circuit or no power supply to the sensor, and will need them to check: 1) The connecting wires between the ECM and the sensor to confirm there are no breaks. 2) The supply power and ground connection at the ECM.
- If the reading is still 0V, then your ECM may be faulty.
If the voltmeter does not return the correct readings for a cold and hot engine and no connection issues exist, you will have to replace the sensor.
Let’s look at how to replace your coolant temperature sensor.
How to Replace an Engine Coolant Temperature Sensor?
The following set of steps is what a mechanic would do to replace the sensor on your vehicle:
- Allow 15 minutes for the engine to cool down to avoid burn injuries.
- Locate coolant temp sensor on the engine block (near the thermostat housing).
- Place a draining pan below the vehicle. There will be coolant leaking out once the sensor is removed.
- Carefully disconnect the wiring connector from the sensor terminal.
- Remove the old sensor by unscrewing it.
- Install the new sensor by screwing it in clockwise. Tighten using a torque wrench in a clockwise direction, to the proper specification, as provided by the manufacturer.
- Connect the connector back to the sensor.
- Start the engine and allow the vehicle to warm up.
- Check the new sensor by looking at the temperature gauge on the dashboard expectantly, in order to monitor any changes, it received from the warming engine.
We can now get to the questions you may have about the sensor in question.
FAQs.
Here’s the answers to some 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 run between $70 – $480 depending on the vehicle model and the labor costs. The costs of the part is between $20 – $80 and the labor can be from $50 – $400.
2. What are the Types of Coolant Temperature Sensors?
There are 3 common types of engine coolant temperature sensors:
- Negative temperature coefficient (NTC) thermistor sensor: These are the most common used sensor. Their resistance decreases with an increase in coolant temperature.
- Positive temperature coefficient (PTC) thermistor sensor: Their resistance increases with an increase in coolant temperature.
- Two-stage coolant temperature sensor: It has two separate internal resistor circuits based on the coolant temperature as the way to increase the accuracy of the voltage signal to the ECU.
3. What is a Cylinder Head Temperature Sensor?
The cylinder head temperature sensor measures the temperature of the engine’s cylinder head instead of being submerged in the coolant. It is used in conjunction with the coolant temperature sensor in some vehicles or is an entirely different standalone sensor in air cooled engines.