Why does the noisy fan on my iMac not stop?

Craig Simons January 31, 2011

When I switch on my iMac the fan kicks in and rages away for ages despite the computer being stone cold. I can usually fix it by restarting a few times, but is there a better way, and why is it doing this? Thanks.

  1. Mbalck482
    April 23, 2012 at 8:54 am

    Thank you Mike!!!!!

  2. Max
    March 28, 2012 at 7:26 pm

    Thank you so much Mike as my mac is nice and quiet again. Also my bluetooth wasn't available because of the fan but now it is working again.
    Thanks again

  3. Prettyboi_swagg1
    March 22, 2012 at 1:09 am

    Damn good advice it work on  the first try. Thanks

  4. Travis
    February 16, 2012 at 11:20 am

    Mike you just saved me! thank you so much dude !

  5. Oliver
    February 9, 2012 at 8:36 am

    Mike you the man! i'm sorted now thank you

  6. Eyner
    February 2, 2012 at 10:12 pm

    Your are a genius Mike :)  thanks Mike.

  7. Ajhender
    January 29, 2012 at 3:50 am

    When i first hooked up my 27" all in one i used the old power cable for convenience. It came on fine but the fan ran in "turbo mode" all the time. When i replaced the power cable that problem disappeared like a bad odor on a windy day.

  8. Fahad Ahmad
    January 22, 2012 at 5:33 pm

    Thank you for your advice, it helped me a lot!

  9. Dion
    November 12, 2011 at 11:57 pm

    Haha i thought i had this problem and spend 3 hours on the phone to a very patient guy at apple, trying all of the above, turned out it wasnt a fan, i had a cd in and forgot about it, it was just reading the disc whenever i started up the mac, haha still feel abit of a fool :D 

  10. fanspeed
    February 20, 2011 at 12:05 pm


    I have a broken fan that generates a grinding noise in my Macbook. I've already ordered a replacement, but it will take a couple of days to arrive. However, I'd like to disable the fan that is generating the noise, for the time being, and I think smcfancontrol can help. But I'm not sure what the arguments you've provided for the utility are.

    /Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 12c0

    I'm assuming the last characters determine the fan speed. If so, which letters/numbers will give me 0rpm?

    I hope you see this, thanks.

  11. Craig Simons
    February 17, 2011 at 1:51 pm

    Okay, tonigh the computer was back to its naughty antics with the fan roaring on full speed. I did the SMC reset and it has worked. Hopefully that's the problem solved. Thanks to all for taking the time to help. Best regards.

  12. Craig Simons
    February 15, 2011 at 2:53 pm

    Thank you all for your excellent advice. I haven't had a chance or the need to try any of it as in the last few days the problem has decided to go away, at least for now. When it returns (as I suspect it will bc it's an on again-off again thing) I will try the SMC reset first (thank you Mike). I will also try the Apple Hardware Test. I don't have the know-how to work on the terminal as described by Fidelis but I have a friend who does, should it come to that. Cheers guys, really appreciate it. Aibek, thank you too, I will be sure to post anything I find out.

  13. Aibek
    February 13, 2011 at 7:43 pm

    Hey Craig,

    Were you able to figure out what was the issue with the fan? Did you fin any of the above tips helpful? Let us know.


    February 1, 2011 at 1:24 am

    Hello, usually when a fan is really noisy is because it is struggling to keep up with the heating. The reason it is struggling is because of dust on the blades. Dust on the blades make the blades heavier and therefore the fan compensates by running at a higher speed. Usually higher speeds mean more noise. Would it be possible for you to check inside your case and clean the fans and the cooler?

    Have you tried the Apple Hardware Test?

    Run the Apple Hardware Test and hopefully It'll show you if something is not running ok. From what I have heard, there have been a lot of cases where the temperature sensor is at fault.

    If you are willing to work on the terminal there is a script that can help you out with the fan speed. Using terminal commands there is a way to force your CPU fan to run at tolerable rpms. This solution should only be used if you know what you're doing and you are absolutely sure the CPU fan on your iMac is indeed out of control. Forcing a low max speed on a healthy CPU Fan with fully functioning sensors can seriously damage your Mac.

    -- Install smcFanControl
    -- In terminal use the following commands
    -- /Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 12c0

    This will set the CPU Fan to a maximun of 1200 rpm

    To go back to default settings(3900 rpm) you can use the following commands:

    /Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 3cf0

    Other fan speeds you can try:
    1100rpm = 1130
    1500rpm = 1770
    3900rpm = 3cf0

    All you need to find out now is a way for this script to run at bootup.

    • Mike
      February 1, 2011 at 4:01 am

      Although I recommend to "repair" the issue to prevent any damage to your system instead of working around it here is how you could solve the "script" thing FIDELIS mentioned.

      You could use a Launch Agent to start this on every boot up. You can use the following online tool to create the file: http://plist.spotmac.de/

      Click on "launchd Quickguide" to see where you have to place the Plist file you are generating there.

      Screenshot how it should look like http://cl.ly/4330123R3g0G1h3R0Q0H
      Of course change the settings after "smc -k..." to match your preferences.

    • Thevnd
      November 2, 2011 at 2:57 am

      can you type the command to set maximum speed for HDD Fan ? Please help me out

      • FIDELIS
        November 2, 2011 at 3:18 am

        Hello, have you downloaded the most recent update for smcfancontrol utility?  Here is the link:


        The reason I ask this is because the newest version has controls for hdd fans.  I believe you can set the max. hdd fan speed there instead of using terminal. 

    • Guest
      March 4, 2012 at 12:43 pm

      super!, was pulling my hair out.. F0Mx is the optical disk one

      • FIDELIS
        March 5, 2012 at 2:18 pm

         I am glad this answer is still helping peopel

  15. Mike
    February 1, 2011 at 12:12 am

    Is it just "noisy" or is it running on what seems to be full speed?

    First one usually indicates either a lot of dust within the Fan and the iMac or the Fans bearing reaching it's end of life.

    If it seems to be on full speed try an SMC Reset:
    1. shutdown your iMac
    2. unplug the power cord
    3. press and hold the Power button for 5-6 seconds
    4. release the power button
    5. plugin the power cable and turn on your iMac

    If this solves the problem there is no explainable reason. Sometimes the System Management Controller (SMC) of the Mac get's stuck with a setting ~ nothing more, nothing less.

    • ~pbs
      September 26, 2011 at 4:47 am

      Great advise!  The sequence of tasks that you recommended worked to stop the fan on my iMac from running full speed without any reason.  :)

    • Jenskoebsted
      November 25, 2011 at 6:01 pm

      Thanks Mike

      you just saved my life, or at least my mac.

      it was very close to be dropped from a moving car


    • Nicholascriticos
      December 18, 2011 at 8:00 pm


      Your a genius !, its like absolute magic , thank you I tried it as well , worked immediately . thanks again .

    • Katielisabeth
      December 19, 2011 at 3:28 am

      Wow worked like a charm! Thanks so much, Mike!

    • Mandybaj
      January 13, 2012 at 7:36 am

      Thanks a ton.You have no idea how much you have helped people.I can also breath a sigh of relief.God Bless!

    • Alex
      February 3, 2012 at 11:02 am

      This is great advice! Thanks a lot

    • Chris Rayan
      March 10, 2012 at 9:11 am

      This was really helpful as my mac had just turned off then we rebooted it and there came the fan noise.Well explained and thanks again!

    • ImfeldC
      April 17, 2012 at 9:26 pm

      Thanks! I used first the German blogs, bt they gave wrong hints. It works fine with your sequence!

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