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Coolant Temperature Sensor questions

please help me understand what the coolant temperature sensor does by answering the following questions for me ?

what exactly does the CTS do and how does it affect engine performance?

can outside weather conditions affect it?s performance, i.e. extreme hot or cold weather?

what causes it to go bad? does it just wear out in time or can it be affected by extreme hot or cold engine temperatures?

when it goes bad how does it affect engine performance? will it cause the engine to stall or not start or just run poorly?


The CTS senses the temperature of the engine coolant within the engine itself. The sensor sends an electrical signal to the computer, which uses this information, along with MANY other inputs, to control the fuel mixture, ignition timing, and other engine parameters.

There may be a separate temperature sensor in the radiator, which controls the cooling fan(s).

Weather extremes should have no affect whatsoever on the CTS.

The sensor can wear out over time. Since the sensor’s probe is actually IN the coolant, old, dirty coolant can corrode the sensor, just as it can corrode the engine. The electrical connection to the CTS is also important, since a dirty, corroded connection can transmit a false signal to the computer.

Extreme engine temperatures should not affect the CTS.

How a bad sensor affects engine performance depends on how it fails. For example: if the CTS tells the computer that the engine is running cool when it is not running cool, the computer will enrich the fuel mixture, as it does at cold start, which will make the engine run rich, possibly damaging the O2 sensors, and wasting gasoline.

Conversely, if it’s 20 degrees outside and you’re trying to start a cold engine, but the CTS tells the computer the engine is already warm, the computer will not enrich the mixture and you will have a very hard time starting the engine, if it starts at all.

A correctly-functioning CTS is very important to engine operation and performance. The CTS can be tested for electrical output under controlled temperature conditions, but since they don’t cost much many people replace them if they even suspect a problem.

It uses the coolant temperature to tell the computer that the engine is cold or warm. Few modern engines have the old chokes, but the computer takes care of that part so it needs to know if the engine is hot or cold, just like the choke on older cars did and ended up doing much the same thing to adjust for the cold engine.

It is designed to handle the temperature of the engine and sub zero winter temps.

I don’t recall the failure mode so I don’t know if they default to cold or hot settings.

On most systems the CTS is a variable-resistance thermsistor.It adds to the base pulse when cold but decreases to zero when the engine warms up.

Do some reading about open loop and closed loop engine operation.

They fail at connections (corrosion) or the wiring fails,rather than the sensor itself,but the sensor can fail.