7 Causes Why Your Radiator Fan Is Not Coming On

A list of the main reasons why your radiator fan won't start

Radiator Fans

You probably already know the purpose of the radiator and the cooling system in your car.

When you are driving fast, the wind is passing through the radiator to cool the coolant down without issues, but when you are driving slow, there is no wind pushed through the radiator. This is why we use a radiator fan.

Unfortunately, the radiator fan can fail and will cause your car to overheat. But what can cause this?

7 Causes of Radiator Fan is not coming on

  1. Broken fuse
  2. Faulty coolant temperature sensor
  3. Broken wirings or bad connection
  4. Insufficient Coolant
  5. Broken radiator fan
  6. Faulty radiator fan relay
  7. Bad fan controller module

Here is a more detailed list of the most common causes of a radiator fan not coming on.

Broken Fuse

Broken Fuse

A fuse supports almost everything electrical in a car. If there is an electrical surge going towards a piece of electronic equipment, the fuse cuts the electric supply to that particular equipment saving it from destruction. This is what we call a blown fuse.

A blown fuse is no big deal, and changing one does not cost a lot of money. If your car’s radiator fan is non-functional, check your car’s user manual and locate the fuse for the radiator fan controller or the fan.

The fan itself often uses a big fuse of around 50A, while there might also be a separate small fuse to the fan control module. Remember that if the fan fuse is blown – there might be a problem with the wirings or the radiator fan.

Faulty Coolant Temperature Sensor

Coolant Temp Sensors

There are often two different systems in different car models. Either your fan control is integrated into the engine control unit or you have a separate fan control module. In both cases, the control units use a temperature sensor to know when to start the radiator fan.

If this temperature sensor is broken, the control unit will not know when to start the radiator fan.

Some cars use separate engine coolant temperature sensors for the radiator fan and the engine control unit.

You need to check your repair manual of which temperature sensor controls the radiator fan and then resistance-measure the sensor with a multimeter to make sure it is functional.

Broken Wiring or Bad connection

Broken Wire

If the fan is not working even when the car is overheating, there might be a wiring issue or a bad connection.

Check the wirings going to the radiator fan from the control unit or relay. Check in the connector plugs for any signs of corrosion. Also, check the connector plugs at the relay and control unit.

Measuring the wirings with a multimeter is often not very effective, as you need to put a load on the wires to see if they are functional. However, as a fast test, you can check with a multimeter if there is coming power to the radiator fan.

Insufficient Coolant

Low Engine Coolant

If your coolant level is low, there is a chance that you will get air in the system, and the coolant temperature sensor will not read the coolant temperature correctly. If the coolant level is low, you need to refill the coolant to optimum levels.

Failing to do so could risk your engine becoming overheated and seized. There is no coming back from a seized engine unless you are willing to spend a lot of money.

Broken Radiator Fan

Radiator Fan

When your radiator fans are not coming on, it can also be caused by faulty radiator fans. The radiator fans have electrical motors inside of them, which will wear out after some years.

You can test the electrical radiator fans by taking a wire from the car battery, unplug the radiator fan connector, and put 12v+ and ground into the connector. This is the fastest and easiest way to test your radiator fans.

Faulty Fan Relay

Bad Relay

Because the radiator fan is often drawing so much power, there is often a relay that is powering the coolant fan. Of course, this relay can get damaged, which will cause the radiator fan to not coming on.

The fan relay is often located in the engine bay’s fuse box, but the best way is to check your repair manual to find where it is located.

Testing a 4 pin relay is often very straightforward. Remove the relay and give 12 volts to pin 30 and 85. Ground pin 86 and check if there is voltage coming out from pin 87. It is even better to connect pin 87 to something that draws a lot of power, like the fan, for example.

Bad Fan Controller Module

Fan Control Module

As I talked about before, some cars have a separate control module for just the radiator fan control. This control module is often installed in the engine bay, exposed to heat and dust. This can make the control module to fail after a while because of corrosion.

Locate the relay and check for any visual damages outside of it. You can often also open up the relay and check for any bad solderings or corrosion. Replace it if you see any problems.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.