Why Does My Radiator Fan Only Work When the AC Is On?

If your radiator fan only works when your vehicle’s AC is on, the most likely culprit is a faulty temperature sensor or thermoswitch. 

What Are Radiator Fans?

Radiator fans are located between your vehicle’s engine and the radiator. They are responsible for keeping your car at a cooler temperature so it doesn’t overheat. There are two types of radiator fans, mechanical and electric. 

What Causes Your Radiator Fan To Only Work When The AC Is On?

Many drivers start to panic when they realize the radiator fan is only working when the AC is on. This can cause the vehicle to overheat. When you turn the AC on, it will automatically turn on the radiator fan to cool down the water in the radiator.  However, when the AC is off, they never turn on any matter how hot the engine gets.

When your radiator fans do not turn on, there is a problem with the temperature sensor or thermoswitch.They are not communicating the message that the vehicle is overheating, so the radiator fans are not being triggered on to cool things down. 

You may be wondering why the radiator fans will come on when you turn the AC on though. This happens because your vehicle’s electric radiator cooling fan gets controlled by a specific thermoswitch. You can usually find this thermoswitch on the side of the radiator. 

When the coolant temperatures rise in the AC, a switch closes in order to start up the cooling fans. The coolant is used to determine the temperature, and anytime its temperature rises to a higher than set level, the fans will be activated to bring the temperature down. Some radiator fans on vehicles with air conditioning will activate from the rising refrigerant levels as well. 

How To Fix A Radiator Fan That Only Works When The AC Is On?

When you notice that your radiator fan is only working when the AC is on, you should make an appointment at your local auto repair shop to have the problem professionally diagnosed. Since you will still likely need to use your vehicle, it’s important that you drive it with care to prevent overheating. 

This issue is most likely caused by a problem with the circuit of the thermoswitch or temperature sensor. There is a chance that if the vehicle is older or has many miles on it that the circuit goes bad. However, another problem that could be affecting the radiator fan is poor wiring. 

Check the thermoswitch cover in the area where the lower radiator hose attaches, because this is where you should find the main thermoswitch that runs the cooling fans. Give the wiring a good examination. You’ll want to make sure it’s firmly connected to the switch. It’s also important to make sure the wires are in good shape. 

If the wiring is in good shape, you may have to book an appointment with the mechanic to see what exactly is wrong and determine whether the thermoswitch or the radiator fan needs to be replaced.

There May Not Be A Problem With Your Radiator Fans

Many drivers are worried because they noticed that their radiator fans are only on when the AC is on. However, this may not be a problem at all if the vehicle isn’t overheating. Those radiator fans will turn on automatically when the AC is on whether they are needed or not. When they are on, they keep the coolant temperature low.

Even when you turn the AC off, the temperature still might not get hot enough to trigger the radiator fan on. In most vehicles, the coolant needs to reach 225 Fahrenheit for the fans to turn on. If you drive with the AC on often, the coolant may not reach that high of a temperature.

A good way to test whether you have a problem or not is to turn your vehicle on without the AC and let it stay idle for roughly 10 to 15 minutes. It’s a better idea to leave your vehicle idle for this test because when you drive, cool air from the wind passes through the car’s radiator.

You should also do this test on a warmer day, because colder temperatures or high winds may also keep the coolant at a lower temperature. After your vehicle has been running for 15 minutes, check to see if the fans have turned on.


When your radiator fans are only working while the vehicle’s AC is on, then it is most likely due to an issue with the thermoswitch or temperature control. It is a good idea to bring your vehicle to an auto repair shop to be sure of the exact problem with the radiator fans.