What is a Coolant Temperature Sensor?

Every driver knows that coolant/antifreeze is needed to keep an engine at optimum temperature. 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 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.

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.

Below, we list the other signs and symptoms which could indicate a CTS issue:

  • Irregular reading of the dashboard gauge (should be 88-90°C when the engine is warmed up)
  • 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

If in any doubt about which component is faulty beneath the bonnet, take your car to a professional mechanic for a complete diagnosis.

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 normally towards the front of the engine, near the radiator or thermostat housing. 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

Coolant sensors are fitted like a spark plug, so you need to unscrew it to remove it. 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. Coolant is likely to leak out at this point, so have the new one ready to replace it, or consider draining the coolant 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 tightening any clips to ensure a good connection with the terminal. 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.

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