Updated: Oct 16, 2021
The fan noise you may hear sometimes after turning your engine off usually comes from the radiator fan.
It's common for some cars to have the fan running for a few minutes after the car is turned off, it helps the engine cool down, especially if it's hot outside. However, if it stays on for longer than 10 minutes, it can sometimes indicate an electrical fault.
Some radiator fans are quiet and some are loud, it depends on the car.
In this article I'll explain why the radiator fan stays on and signs of faults.
Table of Contents:
Why Does the Radiator Fan Stay On?
When the engine is operating, coolant travels through the engine block and its coolant channels and then to the radiator, where heat is dissipated with the help of air flow.
Air flow is provided when the car is moving, helping the radiator dissipate heat, but when the car is stationary you may hear the radiator fan switch on, drawing air through the radiator.
When the engine is switched off, the coolant does not circulate and the heat is trapped, possibly even causing the engine to overheat.
An overheating engine can cause major issues such as engine block or cylinder head cracking and warping, it can also damage other components of the engine.
Sometimes, parts of the coolant and engine may reach different temperatures, placing excess thermal stress on the engine, possible causing cracking and warping.
This issue is exacerbated if you have driven the car hard at high rpms and throttle, increasing coolant temperatures and heat differences even more.
Likewise, if the outside air temperature is hot, the radiator has a harder time dissipating heat, causing increased thermal differences and coolant temperatures.
For these reasons, the radiator fan may stay on for a few minutes after switching the ignition off, helping to dissipate and equally disperse any heat in the coolant system.
How Long Should the Fan Stay On For?
The radiator fan normally stays on for 3-5 minutes after turning off the ignition, though it may stay on for slightly longer or shorter depending on the outside air temperature.
If the radiator fan continues to run for more than 10 minutes, it could be an indication of an electrical problem.
A radiator fan that runs for an extended period of time might drain the battery, causing the car to fail to start the next time you switch on the ignition.
Typically, the radiator fan will only stay on after the ignition is tuned off if the car is driven hard at high rpms with heavy throttle input, or if the outside air is hot or humid.
Signs & Causes of Improper Fan Functioning
There are a number of potential causes and signs of an improperly working fan, causing it to run for an unusually long time after the ignition is turned off.
Low coolant level
Faulty coolant temperature sensor
Defective PCM or ECU
Coolant leaks from the water pump
A leaking heater core
Leaky thermostat housing
A damaged radiator
Worn radiator hoses
Worn or corroded coolant bypass pipe
A faulty radiator fan relay
A short circuit in the electrical system
An abnormal fan operation could have a variety of causes. However, they may be difficult to detect and may be mistaken for typical fan operation.
Signs of Abnormal Fan Operation:
A radiator fan running for longer than 10 minutes after stopping the car
A drained battery after coming back to the car
If you suspect the radiator fan is functioning incorrectly, you can either check for the possible causes yourself or take it to a mechanic for repair.
How to Stop an Abnormally Running Fan?
If the fan has been running for longer than 20-30 minutes, it is very likely that the fan is malfunctioning due to one of the reasons listed above.
In this case, the fan may drain the battery, preventing you from switching on the cars ignition next time you go to drive it.
Step 1: Turn on the car
Turn on the car and let it run for 5-10 minutes, then turn it back off. This will allow the battery to recharge, it will also reset some of the computer systems within the car.
Sometimes this may resolve the issue, if it does not stop the running fan after 10 minutes, continue to the next step.
Step 2: Disconnect the battery
If the fan continues to run for more than 10 minutes after attempting the previous step, disconnect the negative side of the battery connection to prevent the battery from draining.
Below is a video explaining how to disconnect the battery.
You may wish to try and reconnect the battery to see if this resolves the issue.
If the fan persists to run after disconnecting and reconnecting the battery for longer than 3-5 minutes, you can disconnect it again to protect the battery and move on to the next step.
Likewise, if you don't have the tools required to disconnect the battery, move on to the next step.
Step 3: Remove the cooling fan fuse/relay
This is slightly more complicated and requires knowledge of the car but it will work to stop the fan from running but allow the car to continue to drive.
You will need to locate the fuse/relay board, it is usually found either under the engine bay, near the battery, under the passenger side glovebox, or under the steering wheel/dashboard.
The vehicles owners manual should show you where the fuse/relay board is located, the owners manual is usually located in the glovebox, you can also find it online.
Once you have found the fuse/relay board you can remove the correct fuse or relay for the radiator cooling fan, the owners manual should also show you this information.
Below is an example video showing a radiator fan fuse removal for a BMW E90-E91, keep in mind it will not be the same for any other car.
If you don't know how to do this any of this, then you could leave the car battery to drain and the fan will stop, though you will have to get your car recovered or jump started.
Or you can simply switch on the car and drive it straight to the garage, to your home, or to a safe place for repair later on.
If you can gain access to the radiator fans wiring loom connector, you could unplug the connector to the motor, stopping the fan from running, don't attempt this if the fan is running.
Warning: Be very mindful of the coolant temperature if you drive a car without a working radiator fan, if you stop the car, turn the engine off to prevent overheating. Don't attempt to the block the fan or put your hands near a running fan.