Is your car overheating, struggling to start, or showing a jumpy temperature gauge?
That sounds like a coolant temperature sensor issue.
It may be small, but when it fails, it can throw off everything from fuel delivery to cooling, and even lead to engine damage if ignored.
To help you avoid major breakdowns, we’ll explain the signs of a broken coolant temperature sensor, what causes this issue, and how a mechanic will fix it.
This Article Contains
- What Is an Engine Coolant Temperature Sensor?
- 8 Signs of a Faulty Coolant Temperature Sensor
- 3 Common Reasons a Coolant Temperature Sensor Goes Bad
- How to Diagnose a Coolant Temperature Sensor Issue
- How to Replace an Engine Coolant Temperature Sensor
- How Much Does it Cost to Replace a Coolant Temperature Sensor?
- Can You Drive with a Broken Coolant Temperature Sensor?
- Which Coolant Temperature Sensor Does Your Car Need?
What Is an Engine Coolant Temperature Sensor?
The coolant temperature sensor monitors the temperature change of the engine coolant through a resistor circuit. It sends this data to the engine control unit (ECU), the car’s central computer, which uses the information to manage key functions like fuel injection and engine cooling.
How is it done?
The ECU supplies the sensor with a constant reference voltage of 5V, which the sensor converts into a voltage signal based on the coolant temperature.
Most coolant temperature sensors use a negative temperature coefficient (NTC) — meaning their electrical resistance decreases in accordance with a rise in coolant temperature. This results in a reduced voltage output signal.
The ECU monitors these voltage signal changes to manage key engine functions, including:
- Fuel injection and mixing
- Ignition timing
- Variable valve timing
- Transmission shifting
- Radiator fan operation
- Dashboard temperature gauge readings
In addition, 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 that you know how the coolant temperature sensor works, we’ll explain what happens when it breaks.
8 Signs of a Faulty Coolant Temperature Sensor
Here are the key symptoms of a 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 is still warming up, even when it’s already hot.
In response, the ECU will increase fuel injection and adjust the variable valve as well as ignition timing. This can lead to an overly rich air-fuel mixture, delayed radiator fan activation, excessive engine heat, and, eventually, serious engine damage.
2. Illuminated Check Engine Light
A malfunctioning coolant temperature sensor can trigger the check engine light if inaccurate temperature readings disrupt engine performance. It’ll also register an error code you can read using an OBD-2 scan tool.
3. Black Smoke from Exhaust
Your car’s coolant temperature sensor helps the ECU determine the appropriate air-fuel mixture for combustion.
If the ECU detects the engine is cold when it’s not, it may create a richer fuel mixture to mimic a cold start, even when it’s not needed. This richer fuel mixture may not fully combust, resulting in black smoke from the exhaust and poor fuel efficiency.
4. Difficulty Starting the Car
When you cold-start your car, the ECU calculates the required air-fuel mixture ratio depending on the coolant and ambient temperatures. A bad coolant temperature sensor may relay inaccurate data, causing the ECU to produce a lean air-fuel mixture.
The engine will struggle to start without enough fuel for a smooth ignition, particularly when it’s cold outside.
5. Fluctuating Temperature Gauge
The engine temperature gauge on your car’s dashboard gets its input from the coolant temperature sensor.
When the sensor is damaged, corroded, or has a loose connection, it can send erratic or inconsistent signals. These false readings cause the temperature gauge to jump around, even if the actual engine temperature remains stable.
6. Rough Idling
A malfunctioning ECT sensor may cause the engine control unit to inject a fluctuating amount of fuel into the engine. This creates an imbalanced air-fuel mixture, which the engine struggles to burn efficiently, resulting in rough idling.
7. Radiator Fan Problems
Inaccurate data from a bad coolant sensor can cause your car’s ECU to turn on the radiator fan (cooling fan) even when the engine isn’t hot.
Meanwhile, if the sensor fails to signal that the engine’s operating temperature becomes too high, the cooling fan may not turn on at all. This results in insufficient engine cooling, which can cause excessive heat and engine damage.
8. Transmission Shifting Problems
The transmission control module (TCM) uses the coolant temperature data to optimize gear shifts, especially to delay overdrive or high gear engagement while the engine is cold.
However, inaccurate data may cause the TCM to make inappropriate shift decisions, leading to rough shifting, delayed gear changes, or reduced performance.
Wondering what causes these issues in the first place?
We’ll explain below.
3 Common Reasons a Coolant Temperature Sensor Goes Bad
Here 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 ECU may get damaged from corrosion, heat, and contact with the moving parts in the transmission system.
When this happens, the ECU can trigger a sensor-related error code or misread the coolant temperature.
2. Corrosion on Sensor Terminals
The coolant temperature sensor terminals may corrode due to water seepage, causing the sensor to malfunction and resulting in an error code from the ECU.
3. Low Coolant Level and Air Pockets
Insufficient engine coolant and air pockets in your car’s cooling system can negatively affect the sensor readings. In addition, a low coolant level can 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
Diagnosing a faulty coolant temperature sensor isn’t a simple DIY job. It requires specialized tools and a solid understanding of your vehicle’s systems.
A wrong diagnosis could lead to replacing a perfectly good sensor, while mistakes during installation might cause coolant leaks or sensor damage. That’s why it’s best to leave it to a qualified mechanic who can get it right the first time.
Here’s what they‘ll do:
1. Conduct a Visual Inspection
They’ll check your car’s coolant level and inspect the wiring harness as well as the sensor connector for damage or corrosion.
2. Check the Error Codes
The technician will check for error codes using an OBD scan tool. Some codes associated with a faulty ECT sensor are P0115 (Engine Coolant Temperature Sensor 1 Circuit) and P0116 (Engine Coolant Temperature Sensor 1 Circuit/Range Performance).
3. Perform a Voltmeter Test
Lastly, the mechanic will perform a thorough test using a voltmeter. To do this, they’ll:
- Connect the voltmeter’s positive lead to the sensor’s signal terminal and negative to the chassis ground.
- Cold start the engine and check the readings. Depending on the engine’s temperature, the reading should be between 3V and 4V.
- Let the engine warm up to its operating temperature. The voltage reading (for an NTC sensor) should drop between 1.2V and 0.5V.
- If the voltmeter reads 5V, it implies an open circuit. They’ll check the signal terminal connection and the sensor’s ground contact.
- If it reads 0V, this indicates a short circuit or no power supply to the sensor. They’ll check the connecting wires from the ECU to the sensor, as well as the power supply and ground connection for the ECU.
- If the voltmeter still reads 0V, the ECU is faulty.
If the voltmeter’s readings are beyond the range prescribed above for a cold and hot engine and there are no connection problems, they’ll have to replace the sensor.
While testing the sensor with varying engine temperatures is possible, a mechanic may disconnect the sensor and measure its readings at different external temperatures. This entails connecting a multimeter to the sensor, placing it in cold water, and taking the reading (should be 5 to 6.5 volts). Then, they’ll place the sensor in boiling water and take the reading again (0.25 volts).
They’ll then compare these readings to the ones mentioned in your vehicle’s owner’s manual. If they don’t match, they’ll replace the sensor.
Let’s take a look at how they’ll do this.
How to Replace an Engine Coolant Temperature Sensor
Here are the steps a mechanic will take to replace the sensor on your car:
- Let the engine cool down for about 15 minutes to avoid any burn injuries.
- Locate the coolant temp sensor on the engine block (near the thermostat housing).
- Place a draining pan under the car. The coolant may leak out after they remove the sensor.
- Carefully detach the wiring connector from the sensor terminal.
- Unscrew the old sensor.
- Screw the new sensor in the clockwise direction. Tighten it using a torque wrench in the clockwise direction as per the manufacturer’s specifications.
- Reconnect the connector to the sensor.
- Start the engine and let it get warmed up.
- Check the new sensor by monitoring the dashboard temperature gauge to ensure it reflects the change in engine temperature.
- Clear the error codes and bleed the cooling system to remove any air bubbles in the coolant to ensure adequate cooling ability. This involves opening the bleeder valve while pouring coolant into the coolant reservoir until fluid flows steadily from the valve without air bubbles.
Curious to know how much a replacement will cost?
Discover what you can expect to pay below.
How Much Does it Cost to Replace a Coolant Temperature Sensor?
The cost of replacing a coolant temperature sensor ultimately depends on your vehicle’s make and model. Typically, the total charge ranges from $400 to $500.
This includes:
- Parts: $300 to $325
- Labor: $100 to $175
While a sensor issue might not seem like a serious problem, replacing the coolant temperature sensor isn’t something you should avoid.
Read on to find out why.
Can You Drive with a Broken Coolant Temperature Sensor?
While it’s technically possible to drive your vehicle, it’s not recommended as the sensor is essential for the engine’s optimal performance. Bad readings can cause several issues that can reduce engine cooling and lead to engine damage.
It may also cause the ECU to activate your vehicle’s “Limp Mode,” which limits your vehicle’s power and speed to protect the engine. While helpful mechanically, this mode can be dangerous on the road, especially if you need quick acceleration or full control of your vehicle.
You’re probably eager to get a replacement. However, you may be unsure about what kind of coolant temperature sensor your car needs.
Which Coolant Temperature Sensor Does Your Car Need?
There are three common types of engine coolant temperature sensors:
- Negative temperature coefficient (NTC) thermistor sensor: These are the most commonly used sensors. Their resistance reduces in accordance with rising coolant temperature. Most popular car brands, like Toyota, Honda, Ford, General Motors, Volkswagen, BMW, and Mercedes-Benz, use these sensors.
- Positive temperature coefficient (PTC) thermistor sensor: Its electrical resistance increases with a rise in the coolant temperature. These sensors feature primarily on Renault cars.
- Two-stage coolant temperature sensor: It uses two different internal resistance circuits depending on the coolant temperature to deliver a more accurate voltage signal to the ECU. Some BMW vehicles (like the BMW X3), earlier Volvos, and the Holden Commodore use two-stage sensors.
Replace Your Coolant Temperature Sensor with AutoNation Mobile Service
A faulty coolant temperature sensor might seem like a small issue, but if left unchecked, it can lead to serious engine problems and costly repairs.
If you’re experiencing symptoms like rough idling or overheating, it’s best to have a mechanic diagnose and replace the sensor as soon as possible.
Can’t make it to a repair shop?
No problem — an AutoNation Mobile Service technician will replace the coolant temperature sensor from your driveway. We offer easy online booking and back every repair with a 12-month, 12,000-mile warranty.
Contact us before your sensor turns up the heat and your wallet feels the burn!