How to Tell if You Have a Faulty Coolant Temperature Sensor

April 10, 2012

A miscalibrated coolant temperature sensor (CTS) can result in excessive NOx emissions and overheating by masking problems with insuffient coolant.

Car Temperature Reading

The coolant temperature sensor, abbreviated as CTS, in your car is an important device that enables the control unit to give alerts if the engine is overheating or if the temperature within the system is rising for some reason.

The Coolant Temperature Sensor's Function

This device works on the principle of dependence of potential difference in temperature. As the temperature of the engine changes, the potential difference output of the device also changes, and this can be measured by the engine's control unit. Thus, principally, the coolant temperature sensor is a thermistor.

The temperature of a thermistor influences its resistance in an inverse proportion. As the temperature rises, the resistance of the car's coolant drops and this in turn decreases the potential difference output. This voltage output is sent to the electronic control unit of the vehicle, which constantly measures the resistance across the car's coolant. By this continuous monitoring of coolant resistance (and the voltage output), the coolant temperature sensor sends temperature information to the car's engine control unit.

Where to Find the Coolant Temperature Sensor

Different car manufacturers install the coolant temperature sensor differently. However, it is usually in close proximity of the thermostat of the cooling system, or inside it.

There may be two temperature sensors in some vehicles, one to send information from the engine system to the control unit and another from the control unit to the dashboard. However, some vehicles dispense with the need for two temperature sensors by using just one sensor to do both tasks. If there are two sensors, one of them is the coolant temperature sensor while the other is more correctly called the coolant temperature sending unit, which sends information from the control unit to the dashboard of the car.

Symptoms of a Bad Coolant Temperature Sensor

If your vehicle starts to use a lot more gasoline than usual, or black smoke is starting to come from the exhaust pipe, these are indicators that the coolant temperature sensor in your vehicle could be defective, and needs to be replaced. If you start having trouble starting your vehicle after it has reached its normal operating temperature, this is usually a very good sign that you need to have the coolant temperature sensor checked. To confirm, you could run your vehicle through an emissions test. If everything else is in order, failure in this test should be because of a faulty coolant temperature sensor.

Another telltale sign of your coolant temperature sensor not functioning properly is if your engine is overheating frequently. This can possibly happen when the coolant is leaking, causing the temperature sensor to behave erratically.

In many vehicles, a faulty coolant temperature system will trigger a check engine light or service engine light on your car's dashboard. A quick trip to a qualified mechanic with a diagnostic computer will tell you if the check engine light has anything to do with a coolant temperature sensor or not.

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