How to fix: CPU Fan Stuck at 100%

Dennis Faas's picture

Infopackets Reader 'Alan' writes:

" Dear Dennis,

My CPU fan is stuck at 100% full load (same as my chassis fan) - and it's very loud. The fans started running at high RPM shortly after I updated my Asus motherboard BIOS. I tried changing my BIOS fan controls, but that doesn't seem to work. I also downgraded the BIOS firmware by updating to my previous BIOS version, but that didn't help and the CPU fan is stuck, regardless. So, I called ASUS and they said to replace the BIOS chip. I did this and still no luck. Before all this, I could control fan speeds with Silent, Standard, Turbo, or Manual. Now these settings don't work and the fans are running at max speed. Any ideas how I can stop my CPU and chassis fan running at 100%? "

My response:

This is a good question. I have a number of suggestions and I'll list them in order of preference:

  1. Ensure that the Fans are Plugged into the Proper Ports
  2. Clear your CMOS (BIOS) Settings
  3. Adjust Fan Settings in the BIOS
  4. Downgrade the BIOS to another Previous Revision
  5. Determine if the Fans are Faulty
  6. Determine if the Power Supply is Faulty
  7. Manually Override the Fan Speeds using Software (not recommended)
  8. Manually Override the Fan Speeds using Hardware

I'll explain more below.

Ensure that the Fans are Plugged into the Proper Ports

If you accidentally plugged in a chassis fan into the CPU fan header on the motherboard (and vice versa), then the motherboard will not receive the correct fan RPM information and thus may result in fan speeds at 100% load. To correct the problem, ensure that both the CPU and chassis fans are connected to their corresponding ports on the motherboard.

Clear your CMOS (BIOS) Settings

Sometimes the only sure-fire way to reset your BIOS is to use the "Clear CMOS Jumper" on the motherboard. This is especially true in cases where hardware is malfunctioning, or if you don't have a password to get into the BIOS.

To clear the CMOS (and to reset your BIOS), power down the computer, turn off the power supply, open the computer case, then set the Clear CMOS jumper on the motherboard. Usually the Clear CMOS jumper is located near the CMOS battery.

Once the jumper is set to clear, turn on the power supply, then press the power button the computer (this will clear the CMOS). The computer should remain off; if it isn't, power off using the power supply. Then, set the Clear CMOS jumper back to regular operation and power on and see if the fans operate normally. You can watch this Youtube video on how to clear the CMOS using the clear CMOS jumper. Note that when you clear your CMOS, all your BIOS settings will be lost and you will need to set them again.

Adjust Fan Settings in the BIOS

When you flash the BIOS or clear the CMOS, your BIOS settings will be reset. In that case, you need to go into your BIOS, then comb through the settings and look for anything to do with fan settings. Usually the fan settings are in the "Hardware Monitor", "System Health", or "Power" section. You can either set fan settings by RPM or using "smart" pulse width modulation (PWM) settings - known as "smart fan" or "cool and quiet" or "q-fan".

Downgrade the BIOS to another Previous Revision

To help rule out that your BIOS firmware may be at fault, try downgrading to yet another previous BIOS revision. This time, pay close attention to file you're using to perform the upgrade and ensure it is the correct file. I say this because it's easy to flash the wrong BIOS file, as the firmware files are usually ambiguous in their naming schemes.

To downgrade the BIOS, go to the motherboard manufacturer's website and download another BIOS revision lower than the previous one you had. After you flash the BIOS, clear the CMOS settings as I mentioned previously.

Determine if the Fans are Faulty

Most newer computer systems (from 2005 and onward) have fans that use "pulse width modulation" (PWM) hardware controllers to monitor and cool the system. Sometimes the controllers on the PWM fans go bad and need to be replaced. To determine if your fan is bad, take both the CPU and chassis fan out of the computer and run them independently on another motherboard (if you have one) to see if they operate normally. Then, run them at the same time to see if you get the same result. If the fans don't operate as they should, then it's most likely you need to replace one or both fans.

Determine if the Power Supply is Faulty

A faulty power supply can result in strange electronic phenomenon and therefore should not be ruled out. To determine if your power supply, disconnect your old power supply and attach another to your motherboard and see if that makes any difference in the operation of your fans. If it does, then most likely your original power supply is faulty.

Manually Override the Fan Speeds using Software

There is a program called SpeedFan that can manually adjust fan speeds inside your computer (if they operate on PWM). I personally don't recommend that you go this route as the fan speeds won't take effect until Windows is loaded, and even so, it is potentially dangerous to play with such settings (as noted in the program). Also, if the SpeedFan program crashes, then your fans may not work properly. Lastly, SpeedFan has interfered with my RAID card when polling sensors, which then caused my RAID to go out of sync. So, please also note this as a potential hazard.

Manually Override the Fan Speeds using Hardware

If you are still unable to modify the fan speeds using the above methods, you have a few options:

1. Purchase chassis fans with a switch to change speed settings. Run these fans independently from the motherboard.

2. Purchase additional hardware to dial in specific fan RPMs; this is done using a rheostat fan control. KingWin has made a really nice fan speed controller that allows you to control up to 4 fans inside the computer, and displays the temperature on LCD. It also has an eSATA and memory card reader, and it all fits nicely in a 5.25" bay. The controller only works with fans that have the 4-pin header, otherwise known as PWM fans.

3. In the case of the CPU fan that is stuck at 100%, you can purchase a CPU water cooler to significantly reduce the noise as these units are nearly silent at full load. Running the cooler at full load won't hurt anything. Corsair has really great CPU water coolers (I own two of them myself) and have built numerous computers for family and friends with the same water coolers and haven't had any issues. Highly recommended!

Got a Computer Question or Problem? Ask Dennis!

I need more computer questions. If you have a computer question -- or even a computer problem that needs fixing -- please email me with your question so that I can write more articles like this one. I can't promise I'll respond to all the messages I receive (depending on the volume), but I'll do my best.

About the author: Dennis Faas is the owner and operator of With over 30 years of computing experience, Dennis' areas of expertise are a broad range and include PC hardware, Microsoft Windows, Linux, network administration, and virtualization. Dennis holds a Bachelors degree in Computer Science (1999) and has authored 6 books on the topics of MS Windows and PC Security. If you like the advice you received on this page, please up-vote / Like this page and share it with friends. For technical support inquiries, Dennis can be reached via Live chat online this site using the Zopim Chat service (currently located at the bottom left of the screen); optionally, you can contact Dennis through the website contact form.

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