Cooling Fan Comes On When Car Is Cold [Reasons + Solutions]

The cooling fan comes on when car is cold due to several reasons. A stuck cooling fan relay, damaged temperature sensor, and cooling system thermostat may be stuck closed are some of those caveats behind this issue.

Turns out, an unplugged thermostatic switch or activated defrost function can also be a reason for causing the cooling fan to come on even when the vehicle is cold. 

Below, we will walk you through every troubleshooting step to solve those issues. So, stay in tune with us till the end. 

Cooling Fan Comes On When Car Is Cold [Why + How To Fix]

It’s time to break down those culprits that make the cooling fan stay on when the car is cold. 

Note: You can also read how to fix the cooling fan comes on with ignition.

1. Cooling System Thermostat Remains Stuck Closed

The cooling system thermostat on a car is specially designed to bring your engine up to normal operating temperature. 

If it gets stuck closed, it will overheat the engine in a short time. So, the cooling fan coming on while the engine remains cold could be normal if the AC is on. 

cooling system thermostat remains stuck closed 

Several reasons are responsible for a sticking thermostat. For example, incorrect thermostat installation and installing the wrong thermostat are the culprits that cause the cooling system thermostat to remain stuck closed. 

Besides, manufacturing defects or a massive level of corrosion can also cause the thermostat to get stuck closed. 


Before doing anything, we recommend you perform this test first: Remove the thermostat and slip a thin piece of metal in between the poppet and the body. It will grip the metal piece with the thermostat closed. 

Then, hang the thermostat and suspend it in a pot of water on your kitchen stove.

Now, heat the pot, and the thermostat should open and fall to the bottom of the pot before it boils. If not, it’s time to replace the thermostat. Fortunately, the thermostat is cheap and it costs you around $50.

We highly recommend you replace the thermostat with a new one and ensure you get a similar thermostat that matches the old thermostat. 

On the other hand, if the thermostat falls, you should look for other issues like a clogged hose, an air bubble in the cooling line, or a blocked radiator. 

2. Temperature Sensor Gets Damaged

A damaged temperature sensor is another possible reason that can cause the cooling fan to come on even though the car is cold. If the coolant temperature sensor goes bad, you will observe the following symptoms: 

damaged temperature sensor
  • Intermittent or faulty electrical cooling fans
  • Overheating engine 
  • Reduced fuel economy 

Electric short circuits, temperature sensors exposed to excessive temperature, and bad electrodes are some common reasons that cause the sensor to go bad. 


There is no option except to replace the coolant temperature sensor if it fails. You can change this part by yourself.

Just get a new temperature sensor that matches the old one and follow the steps mentioned in this video to complete the replacement: 

3. A Stuck Cooling Fan Relay

A stuck relay can also be responsible for the cooling fan coming on even though the engine is cold. There are a lot of reasons that can cause the relay to stick. Among them, is micro-welding of the contacts is caused by arcing when the contacts close or open.

cooling fan relay 

It can happen when the current being switched is within the rating of the relay. You can easily determine whether there is a problem with the relay or not by unplugging the connector from the temp sensor. If it keeps the fan running, the relay is at fault. 


The simple way to get the relay out of the stuck position is to knock the relay with a wrench. Just gently hit the relay with a wrench and it will solve the issue. Either way, you can replace the relay to fix it.

4. Thermostatic Switch May Remain Unplugged

Yes, an unplugged thermostatic switch could also be a culprit behind this issue. We guess you or your mechanic may forget to plug in the thermostatic switch after replacing the timing cover chain gasket. 

Either way, the thermostatic switch gets defective. If the switch or its wiring is at fault, the fan will come on to prevent the motor from overheating. 


The solution to this issue is simple. Locate the thermostatic switch and ensure you plug it in. You can find this switch at the bottom of the radiator. If it remains unplugged, just plug it in to reset the system. 

On the other hand, if the thermostatic switch gets defective, you should replace it. We recommend you hire a professional to make the replacement if you are not mechanically inclined. 

5. Defrost Function Gets Activated

The cooling fan seems to run all the time even when it’s cold! If this is the issue you are going through, we bet the defrost function of your car gets activated. This function enables under a specific outside temperature to prevent windows from fogging up. 


We recommend you try to set the temperature and blower speed manually. You can have the air on the vent or feet, or a combination of both to turn off the AC. Otherwise, you can bring your car to a mechanic to deactivate the defrost function. 


What temperature should a cooling fan come on?

Most cooling fans on a vehicle come on when the engine operating temp reaches 200 to 230 degrees. To check at what temperature the cooling fan is coming, just turn off the AC and keep the engine running until it reaches the normal operating temp. 

What are the symptoms of a faulty cooling fan switch?

Damaged signal wiring, check engine light is on, and an overheated engine is some common signs of a faulty cooling fan switch.

What sensor controls the radiator fan?

The Engine Control Unit or ECU controls the radiator fan, which gets signals from the coolant temperature sensor.


Throughout this entire article, we broke down everything that causes the cooling fan to run constantly even when the engine is cold. So, follow our guidelines and it will solve the cooling fan issue.

If the problem still persists after trying out those troubleshooting steps we mentioned, it will be better to bring your car to a certified mechanic. Good Luck.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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