8 Symptoms of a Bad Coolant Temperature Sensor

(Updated on February 19, 2021)

Knowing the temperature is very helpful. If you are going outside, you look and see that it’s 28 degrees and you should grab a warm hat. While your car doesn’t wear cozy hats, it’s crucial that it constantly monitors the temperature of the engine while it’s on.

That’s the job of the engine coolant temperature sensor (CTS or ECTS). Coolant is also known as antifreeze, the fluid that helps keep the engine at an optimal operating temperature.

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There are several things that the vehicle can do to change the temperature when needed, so the temperature data that the CTS sends to the ECU (car’s main computer) is critical.

Some vehicles also have a cylinder head sensor (CHS) which sits at the top of the cylinder and is not affected by loss of coolant since it isn’t submerged by coolant like the CTS is. This makes the CHS more reliable than the CTS.

How Does a Coolant Temperature Sensor Work?

coolant temperature sensor replacement cost

The CTS uses electrical resistance to measure the temperature, which means that the CTS is a thermistor. The resistance (opposition to electrical flow) of the sensor changes proportionately with temperature – as the temperature increases, the electrical flow also increases.

The ECU sends the electrical signal through the CTS, measuring the voltage drop. This converts information about the electrical flow into a temperature reading.

With that information the ECU adjusts the fuel injection, ignition timing, and electric radiator cooling fans to maintain an optimal temperature. If the engine is cold, the ECU directs the air/fuel mixture to be more rich, or a higher proportion of fuel for the amount of air entering the engine.

If the engine starts to get too warm, the ECU will kick on the radiator fans. This is normal behavior when you are sitting at a long stop light on a hot day, for example. Some cars shut off the engine if it’s overheating to protect from engine damage.

The temperature information is also sent to the dashboard gauge which is usually next to the fuel gauge.

Symptoms of a Bad Coolant Temperature Sensor

All parts wear out eventually, and this sensor is no exception. It’s critical to address problems with the cooling system, because if the vehicle ends up overheating it can cost you an engine (which is very expensive and time consuming to repair).

1) Overheating Engine

engine overheating symptoms

An overheating engine should give off several warnings like a high temperature reading on the dashboard gauge and sometimes white “steam” coming out from under the hood (this is boiling coolant, which means it’s leaving the system – that’s bad!).

Not having enough coolant is a problem. Leaking coolant can also cause the engine to overheat if there is not enough reserve to properly cool the engine.

2) Poor Engine Performance

slow acceleration

If the sensor is faulty it may send incorrect temperature information to the ECU which can lead to strange engine behavior, like overall “weakness” or sluggishness.

If the vehicle lags going uphill, is slow to accelerateidles roughly, or is difficult to start especially when it’s already warmed up, it’s worth checking the CTS.

3) Increased Fuel Consumption

high fuel consumption

You may see that your fuel economy worsens considerably if the sensor is bad because the computer may be directing too much fuel to be injected into the cylinders.

4) Black Smoke from Exhaust

black smoke from exhaust

For the same reason, the vehicle may run too rich which makes the excess fuel burn up in the exhaust and causes other drivers to glare at you.

5) Failed DEQ Emissions Test

emission test failure

If too much fuel or an abnormal amount of byproducts are being expelled due to inefficient combustion, it will show up on an emissions test as something that needs to be fixed.

The CTS may be the culprit, though there are several sensors and gaskets that should be checked.

6) Inaccurate Temperature Gauge

temperature gauge is high

If the engine temperature reading on the dashboard gauge seems wrong (for example, if the gauge reads “cold” when the car is thoroughly warmed up), it may be getting wrong information from the coolant temperature sensor.

7) Check Engine Light is On

check engine light

The dashboard “check engine” light illuminates when the ECU detects a problem and stores a code. If you see this along with any of the other symptoms, it’s worth checking the CTS.

8) Cabin Air Conditioner Stops Working

air conditioning

Many vehicles will put the car into “fail-safe” mode if overheating is detected. This may turn off the engine, run the engine cooling fans continuously, and disable the interior AC to enable the car to dissipate heat from the engine more effectively.

Coolant Temperature Sensor Replacement Cost

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The sensor is usually fairly easy to access, often on the radiator or in or near the thermostat. It’s a good idea to have a trusted mechanic take care of the diagnosis and replacement since the coolant should be refilled and air bubbles removed after the part is replaced.

Some coolant temperature sensors may be found in the engine block, or in part of the cooling system plumbing.

The sensor itself is usually between $25 and $60 depending on the vehicle, and labor is between $150 and $300, though it could cost more if other things need to be done to the vehicle.

Some vehicles do have a second coolant temperature sensor, located elsewhere in the engine bay or in the radiator.

If you choose to try to replace the CTS yourself, always practice safety first and remember that the coolant system is under a great deal of pressure when hot. Never remove the coolant cap or anything else in the system until the car is completely cool!

If you see any of the above symptoms, check the sensor itself and all connections and wiring between the CTS and the ECU to make sure there isn’t another issue along that line of communication.

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