CTS coolant temp sensor
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How the coolant temperature sensor works

In this page we will show you how to diagnose and replace a bad CTS ( Coolant temperature sensor ). First we will explain how a coolant temperature sensor works.

1- How does the coolant temperature sensor "CTS" work?
 The coolant temperature sensor is a thermistor ( a resistor which varies the value of its voltage output in accordance with temperature changes. ). The change in the resistance values will directly affect the voltage signal from the water thermosensor. As the sensor temperature decreases, the resistance values will increase. As the sensor temperature increases, the resistance values will decrease.
The coolant temperature sensor lets the engine control computer know what the engine temperature is by gathering  information from the engine coolant temperature.
 The most common coolant temperature sensor location is near the thermostat housing, sometimes the computer uses the same sensor to operate the temperature gauge in your instrument cluster, depending on the car make and model.

 If your vehicle is equipped with what seems to be two coolant temperature sensors , your car has a coolant temperature sensor and a coolant temperature sending unit, the coolant temperature sending unit operates the temperature gauge in your instrument cluster and the coolant temperature sensor sends the signal to the engine control computer to transmit the correct engine temperature, the way to recognize which one is  the sending unit and which one is the actual coolant temperature sensor is to unplug the electrical connector and see how many wires are attached to it, the CTS has two terminals, while most temperature sending units only have 1.

2- How do I know if the coolant temperature sensor in my car is bad?

  There are several ways to know if the coolant temperature sensor is malfunctioning, if the sensor is bad it will trigger a trouble code and the check engine light in the dashboard will come on, you can retrieve the engine code and see if it is related to the coolant temperature sensor, even if the engine control computer doesn't store a trouble code, there is another way to suspect a bad coolant temperature sensor : If your vehicle starts using more fuel than usual, starts having trouble starting when the engine reaches normal operating temperature or you notice black smoke coming out from the exhaust tail pipe, it is very likely that these symptoms are related to a bad coolant temperature sensor. We have included in this page  detailed information enhanced with pictures about how to diagnose and replace a bad coolant temperature sensor. 

Related video:

 The vehicle we are working on is a 1997 Kia Sportage, equipped with a 2.0 L DOHC engine. Even though the steps shown in this page are related to a Kia Sportage, the same steps apply when troubleshooting a coolant temperature sensor in most makes and models including Acura,BMW,  Buick, Cadillac, Chevrolet, Dodge, Ford, Honda, Jaguar, Lincoln, Mercedes Benz, Nissan, Porsche, Pontiac, Saturn, Toyota,Volkswagen and more.









The Kia Sportage we are working on had developed the symptoms described earlier in this page, ( Decreased fuel efficiency, black smoke and trouble re-starting at normal operating temperatures ). We used an Auto X-ray to access the trouble codes stored in the engine control computer and we found  the code 117 "Engine coolant temperature low input" .

This particular scanner allowed us to read the parameters with the engine running, and we were able to see that in fact the coolant temperature sensor was not sending the correct signal, it was stucked on -40 degrees F.

 If you don't have access to this kind of scanner but you would like to know how to test the coolant temperature sensor with a volt/ohm meter, keep reading, the first step is to find the coolant temperature sensor, in this particular car the CTS is located below the thermostat housing in the front of the engine.
 To gain access to the sensor it is necessary to remove the air duct that supplies cold air to the air filter housing, this air duct is attached to the radiator by two bolts and the air inlet hose is secured by a hose clamp, you need to loosen the hose clamp, remove the two bolts that attach the air duct to the radiator and remove it from the vehicle. Next unplug the electrical connector from the coolant temperature sensor.

 With the air duct removed, the electrical connector unplugged, and the engine off, proceed to check the coolant temperature sensor with a standard ohmmeter by setting it on ohm x 1000, connect the positive and negative probes to both terminals on the CTS as shown in the picture.

- The normal resistance on an engine with 50 to 80 degrees F = 2,200 to 2,700 ohms.  As the sensor temperature DECREASES, the resistance value will INCREASE.

 If you are going to check the sensor with the engine running, make sure that the testing equipment is away from all moving parts like radiator fan and accessory belts.

 - Once you know that the coolant temperature sensor is bad by checking it using the steps described earlier, the next step is to replace it, because the coolant temperature sensor is screwed directly to the engine and has direct access to the engine coolant, you need to drain the antifreeze in a suitable container by unscrewing the radiator drain valve.

 - Next, using either a wrench or a socket, remove the coolant temperature sensor by unscrewing it in the same direction you would unscrew a regular bolt.

 In this particular coolant temperature sensor, it is not necessary to add thread sealant to the coolant temperature sensor because it is equipped with a copper washer to provide a tight seal and avoid a coolant leak.

Depending on the vehicle you are working on, you may need to add either a teflon tape or thread sealant  to the threads on your coolant temperature sensor to provide a tight seal.