Every driver knows that coolant plays an important role in helping the engine operate at the correct temperature. Working alongside the thermostat, radiator, water pump and cooling fans, it helps transfer excess heat away from the engine. But how do cooling systems know when the engine is operating at the right temperature?
In this guide, we take a closer look at coolant temperature sensors, offering information on what they do and how they work, and step-by-step advice on diagnosing and replacing a faulty sensor yourself.
What is a Coolant Temperature Sensor and How Does It Work?
A coolant temperature sensor (CTS) (also known as an ECT sensor or ECTS (engine coolant temperature sensor) is used to measure the temperature of the coolant/antifreeze mix in the cooling system, giving an indication of how much heat the engine is giving off. The sensor works with the vehicle’s ECU, continually monitoring the coolant temperature to make sure the engine is running at the optimum temperature.
To get an accurate reading of the current engine temperature, the ECU sends a regulated voltage to the CTS. The resistance of the sensor varies with temperature, this is how the ECU can monitor temperature changes. The ECU uses this reading to calculate the coolant temperature, and from there adjusts the fuel injection, fuel mix, and ignition timing, and controls when the electric cooling fan is switched on and off. This information is also used to send an accurate reading of the engine temperature to a gauge on the dashboard.
How to Replace a Coolant Temperature Sensor
Replacing a coolant temperature sensor is a simple process for anyone who is familiar with the components of an engine, and can be done right on your drive. The steps below demonstrate how to change a faulty coolant temperature sensor in your car.
Step 1: Locate the Sensor
The CTS is typically located in the engine’s coolant passages, commonly near the thermostat housing, coolant outlet or cylinder head. Because it’s a small component and is often located lower down inside the engine bay, you may need to use a light or torch to find it. Removing the engine cover can also help you find it if it’s located near the thermostat housing.
Step 2: Remove the Connector Cable from the Terminal
The CTS is connected to the ECU by a connector, which you’ll need to unfasten and remove. Do this carefully, as the plastic connector and wiring can often be brittle and will need completely replacing if it breaks. Disconnect the connector, and set the cable aside so it’s out the way.
Step 3: Loosen and Remove the Old Sensor
Many coolant temperature sensors are threaded into the cooling system, although some designs use retaining clips instead. Using a deep socket and ratchet, carefully loosen the sensor in an anticlockwise direction, without applying too much pressure. A squirt of release spray can help free up stuck sensors. Once the sensor is loose, unscrew by hand and remove it from the socket. Ensure the engine is completely cool before removing the sensor. Depending on the vehicle design, some coolant may escape when the sensor is removed, so have a replacement sensor and suitable coolant ready if required.
Step 4: Install the New Sensor
Using a rag or cloth, clean the area of dust and debris which may impact on the performance of the new CTS. Set the new sensor in the threads and twist clockwise by hand, making sure the sensor is well-seated in the socket. Then, using a torque wrench, tighten the sensor to the amount specified in the manufacturer’s instructions.
Step 5: Reinstate the Connector Cable
Once the new sensor is in place, the only thing left to do is reconnect the cable. Make sure the connector is clean and free from debris, then carefully plug it into the new sensor, carefully pushing it into place until the locking tab clicks securely into position. To make sure the new sensor is working, start the engine and as it warms, monitor the temperature gauge on the dash to make sure the correct temperature is being sustained.
Can you Drive with a Bad Coolant Sensor?
While it is physically possible to drive with a bad coolant sensor, we would strongly advise that you do not. A faulty sensor can lead to overheating issues throughout the car’s components which can not only be costly, but dangerous.
We would advise getting this fixed as soon as possible by a reputable mechanic.
How to Test a Coolant Temperature Sensor
Check the general condition
Start by having a look at the unit itself to check its condition, as sensors/gaskets/connectors can develop cracks with extended use and continual temperature cycling. The CTS is usually found at the front of the car, near the thermostat housing, or on the radiator. While a visual check can help to diagnose some faults, not all problems with a CTS show visible symptoms.
Inspect the wiring
As well as the general condition, ensure that the wiring is correct. Ensure that the wires are in good condition with no fraying cables and that they aren’t loose. An issue with the wiring may be fixed as simply as just reconnecting some wiring.
Use a multimeter
Use a multimeter to assess whether the problem is with the sensor’s wiring harness. Turn the ignition key without starting the engine, connect the red line of the multimeter to terminal two and ground it with the black line. Compare the measured voltage against the manufacturer’s specifications. The expected value can vary depending on vehicle design and sensor type.
Use an ohmmeter
The actual sensor should be tested with an ohmmeter. Disconnect the sensor from the wiring while the engine is off. Attach the ohmmeter to the terminals on the sensor. Make a note of the resistance. Reconnect the wires, run the engine for a couple of minutes and then turn it off. Repeat the first steps and make another note of the resistance. The resistance should change as the sensor temperature changes. Compare the readings against the resistance-to-temperature specifications provided by the vehicle manufacturer.
How to Reset a Coolant Temperature Sensor
Coolant temperature sensors use temperature-sensitive electronic components to send information to the vehicle’s ECU and cannot be manually reset. If a sensor has failed, it typically needs to be repaired or replaced.
How to Spot a Faulty Coolant Temperature Sensor
Like any component under the bonnet, coolant temperature sensors can develop faults over time. A faulty sensor can lead to a range of problems developing, including overheating and poor engine performance. That’s why it’s important to know how to spot the signs of a faulty or failing temperature sensor, before it can cause further problems which could prove more expensive to fix.
Below, we list the other signs and symptoms which could indicate a CTS issue:
- Irregular reading of the dashboard gauge (an unusually low, high or fluctuating temperature reading compared with the vehicle’s normal operating temperature.)
- Overheating engine (highlighted by dashboard gauge)
- Check Engine Light alert on dashboard
- Rough engine sound while idling
- Limited performance (caused by ECU miscalculating fuel rich mixture)
- Poor fuel economy
- Difficult cold starting
If in any doubt about which component is faulty beneath the bonnet, take your car to a professional mechanic for a complete diagnosis.
Prestone car maintenance products are tested in extreme conditions, so you can be confident they offer long-lasting performance for everyday driving. For more information, visit the Prestone homepage.