When A Coolant Temperature Sensor Goes Bad

It is located in the coolant passage of the internal combustion engine near the thermostat valve or in some vehicles even inside of it. The sensor keeps a tab on the operating temperature and alerts car’s computer when there is a potential overheating problem. Typically the sensor consists of two wires – one supplies a 5 volt signal to the computer and the second is connected to the ground. When the temperature is high, the internal resistance across the CTS drops. Powertrain Control Module (PCM) of the vehicle reads the voltage drop across the signal circuit and uses this input for controlling engine functionality. Some engine functions such as air-fuel ratio, fuel injection timing & ignition timing are dependent on the engine temperature. Based on the data received from engine temperature sensor, the PCM will control some of the functions of engine such as the fuel mixture, ignition timing, switching on / off of the engine cooling fan etc. The PCM also controls the temperature gauge or the temperature indicator in the dashboard.

Bad Engine CTS Symptoms

The coolant sensor can go bad for many reasons leading to engine overheating and drivability problems. Problems with other systems may also impact the working of sensor. For instance, when there is a coolant leak the sensor may not respond because there is no coolant around it to record the temperature. The temperature gauge will not show any warning light even when engine gets overheated. The sensor can have damages or cracks due to corrosion, leakage around the thread or broken wire connectors. This requires thorough testing and visual inspection to diagnose the issue. If the fuel consumption is high, engine is overheating frequently or if the cooling fan fails to run normally then you may need to inspect the engine cooling sensor for damages, cracks or leaks. If you notice engine temperature light is continuously on without changing then you can suspect a problem with cooling system. In most cases, a problem with CTS will activate the check engine light. If the problem is simple you can replace the bad CTS by following some simple guidelines. If the issue is a complicated then it is better to take the vehicle to the service centre. Also, make sure you don’t drive the vehicle if the engine is overheated with high temperature indicator ‘H’ on in red color.

Diagnosing Bad CTS

It is risky to run the vehicle with faulty temperature sensor because it may damage the engine and escalate other issues. A scan tool can be used to scan for any error codes of a bad CTS. Another way is to check voltage using volt-ohm meter. Turn the key to the ON position and use volt-ohm meter to measure 5 volts DC at the terminal. Now start the engine and allow it to warm to the normal operating temperature. Set the meter to Ohms to get the resistance reading. The resistance should be between 1.5 to 2 ohms. If there is no reading in the meter then we can assume the sensor is bad. As the engine becomes warm the resistance should drop. If it doesn’t decrease than the sensor is bad and needs a replacement.

Replacing Bad CTS

Once you are sure the sensor is causing an issue, you can replace it following the standard procedure. Since the placement of sensor is different in different car models, first identify its location. If necessary remove large components to reach it. Sometimes, you may also need to dispose some coolant to make it easy for the removal of sensor. Remove electrical wiring harness, replace the bad sensor with a new one and make sure it is sealed properly to avoid leaks. If necessary refill the radiator with the new coolant mixture.

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