Fans going crazy, CPU 100% idle


Recommended Posts

I'm going insane over the behavior of my 27-inch, medio 2010 iMac. The CPU fan continues to spin up even though the iMac doesn't do a thing. I've reinstalled OS X Lion multiple times, I reinstalled OS X Mountain Lion several times. I did PRAM-resets more times I can count, same goes for SMC resets. Today I even disconnected the iMac, laid it down on my bed and cleared all the vents with the vacuum cleaner on the weakest setting. The other day a friend of mine walked in and immediately asked why my iMac was making so much noise.

The first picture shows the CPU Fan right after startup. The second less than two hours later. I haven't done a thing with the iMac except play music and browse the web a bit.

How on Earth is it possible the fans spin up, sometimes over 4000 rpm, when the CPU sits next to idle most of the time? The worst part is they won't power down once the iMac cooled off. There are no optical drives in the SuperDrive either.

The measurements are in Celsius. The room temperature averaging at 20,5 degrees throughout my apartment. It's only 14 degrees here in Amsterdam.

post-128385-0-15321200-1348764232_thumb.

post-128385-0-38335300-1348764664_thumb.

post-128385-0-02210400-1348765019.png

Link to comment
https://www.neowin.net/forum/topic/1108523-fans-going-crazy-cpu-100-idle/
Share on other sites

assuming your temps are in Celsius it is spinning up because the chip is hot. Is there anything to justify the chip being hot? Dust on the HSF, high ambient temp, etc?

I can't really say for sure how much dust has accumulated inside the iMac itself, but judging by how little there was directly underneath the grill I seriously doubt it's a lot. I tried looking inside with a flashlight but everything seemed clear.

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

Did you recently install an SSD?

http://forums.macrum...d.php?t=1235234

That actually affects the hard drive fan, not the CPU fan (at least it did in my Late 2010 iMac). Also, it happens a lot sooner than 2 hours after boot. :p

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

It's an iMac. It's normal for things to run a little bit hotter.

Did you recently install an SSD?

Everything is stock. No modifications whatsoever have been made to the iMac after I unpacked it. Also, the SSD issue wouldn't affect the CPU fan, rather the HDD fan.

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

Okay, this is something I can work with. Is there any way of checking what the GPU load is?

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

That's EXACTLY what ****es me off. The fans won't slow down once things are cooling off.

UPDATE:

Okay this is incredibly interesting. I'm now 100% sure this is indeed a software issue, rather than a hardware related one. For whatever reason OS X Mountain Lion is telling my iMac to increase CPU fan speed in order to cool off. But apparently this is a one-way street. As time progresses the fan speed just continues to go up until it hits a maximum of about 4150 rpm. Like jamesyfx noticed they it will continue running at its maximum until I do a full shutdown. So for whatever reason OS X Mountain Lion won't tell my iMac to reduce fan speeds once it cooled off.

After a lot of Googling I noticed other people having the same issue with OS X Mountain Lion. Not just iMacs but MacBook Pros too. One person came with the suggestion to temporarily download and install iStat Menus 2, set Fan speed control to "Medium" (it's on of the default "rules") and then return to "Default". In literally an instant my iMac's CPU fan would drop from its maximum of ?4150 to a quiet 1940 and then continued to drop.

Check out the below screen shot and note the uptime: Over 6 hours and a regular CPU fan speed of 1885 (it's still dropping as I type).

So what's going on here people?

post-128385-0-90926700-1348790216_thumb.

It does a similar thing on my system, CPU fan speed drops down to a steady 1190rpm when I set to default.

Seems like it kicks the default behaviour back in once you set it as such.

My iMac is a completely different model to yours, so the issue must be software related.. and I think using this utility has proven that.

Yeah that's exactly what seems to happen. iStat basically slaps OS X on the wrist telling it do proper fan control. After a while CPU fans would indeed drop to an average of 1190 rpm on my model as well. Here I was thinking my CPU/GPU fan was constantly triggered because it couldn't handle the new OS X Lion/Mountain Lion graphics effects. But that's not it either.

Anyway, it seems like you can uninstall iStats Menu from your system until you do a SMC reset.

I'm not familiar with Mac since i never owned one but why does OSX has to control the fan ?

I would never ever trust any OS to control the fan speed of my computer. Right now i have a fan controller but if i would not have one i would let my Motherboard control the fan speed or at worst install my MB software (or a 3rd party alternative like speedfan) to control them.

Here's an interesting blog post about the issue. You probably have already read it since it was in the first 10 results of a google search but i still post it just in case.

http://gigaom.com/ap...lion-heres-why/

Spotlight reindexing doesn't take days or weeks.

  • 2 weeks later...

Maybe this helps:

http://www.hddfancontrol.com/

OR:

Symptoms

After plugging in the iMac and turning it on, the fans (blowers) may be noisy and appear to run at full speed. In this event, shutting the iMac off and turning it on again wouldn't fix the issue.

Resolution

If you press the iMac power button while you are inserting the power cord, the iMac will enter a mode in which the fans (blowers) run at full speed. In order to correct the issue, reset the SMC and then start the computer by pressing the power button after the power cord has been fully inserted.

The iMac should now start up and operate as expected.

http://support.apple.com/kb/TS1433

AND:

First download and install smcFanControl 2.2.2. Set it up to provide (still quiet) minimum fan speeds of 2200 rpm for the HD and 2800 rpm for the CPU, leave the Optical Drive fan at 600 rpm to minimize dust accumulation (or just tweak it up if/when you will be using it much) ....

http://hints.macworl...110606033134359

UPDATE:

Okay this is incredibly interesting. I'm now 100% sure this is indeed a software issue, rather than a hardware related one. For whatever reason OS X Mountain Lion is telling my iMac to increase CPU fan speed in order to cool off. But apparently this is a one-way street. As time progresses the fan speed just continues to go up until it hits a maximum of about 4150 rpm. Like jamesyfx noticed they it will continue running at its maximum until I do a full shutdown. So for whatever reason OS X Mountain Lion won't tell my iMac to reduce fan speeds once it cooled off.

After a lot of Googling I noticed other people having the same issue with OS X Mountain Lion. Not just iMacs but MacBook Pros too. One person came with the suggestion to temporarily download and install iStat Menus 2, set Fan speed control to "Medium" (it's on of the default "rules") and then return to "Default". In literally an instant my iMac's CPU fan would drop from its maximum of ?4150 to a quiet 1940 and then continued to drop.

Check out the below screen shot and note the uptime: Over 6 hours and a regular CPU fan speed of 1885 (it's still dropping as I type).

So what's going on here people?

I'm confused. Did that software package end up solving the problem for you or is it still an issue?

Off hand I would say the temp gauges are wrong, without touching the cpu or the motherboard touch the bottom of the heat sink(while the computer is on) and see if its hot. That feels like the CPU is over heating(which would cause the 100% cpu usage).

I have no idea what you're talking about or why you think there is 100% CPU usage. :/

  • 2 weeks later...
  • 2 years later...

Hi there, old thread, but hopefully someone is still around.

I have an iMac 27 late-2009, 3.06 Ghz, 8gb RAM, running OS 10.8.5, and all the hardware is original (no HD swap...). So, it has a similar problem: it takes about an hour for the CPU fan to gradually speed up from the initial approx 1200 RPM to around 4000 RPM. Then, all I have to do is to put it to sleep for enough time to let the fans stop spinning (that's not even 5 seconds) and then wake it up to see the CPU fan at normal speeds. And here it goes for another hour of gradual increase... See the screenshot of the iStats Menu, the graph shows the increase in the last hour, and the three previous increase/sleep patterns... The other two fans are just fine. All of that time, the iMac is at idle, nothing running at all. My iMac isn't my main computer, so i kinda left the problem as it was, spending an hour or so every couple of months trying to solve the issue, but after these 5 years of fruitless efforts, I can't believe I haven't solve it yet! Even though I get answers a lot from forums, here's my first post in one of them (ever!), in hope it will help.

 

Here are the solutions I tried already: smcFanControl, HDD Fan Control, iStats Fan Rules... Nothing seems to be able to override the fan speeds, nothing at all! I vacuumed the air input and output... And of course, I did the SMC reset. I try to remember all the other things I tried to do, but it gets confusing after those years of diverse problem solving. I did many upgrades (and even downgrades) too but I doubt it has anything to do.

 

Even though I never opened the iMac's case (I DID open the bottom latch to replace RAM memory at some point, comes to my mind... but that's not the back case) I wonder if it has anything to do with some hardware that would just be misplaced at the factory of because of a shock while moving the computer around... 

 

Anyone has any idea? Anything I could try?

 

Screenshot:

https://www.dropbox.com/s/vfvkr1zordrbznb/Screen%20Shot%202015-01-14%20at%2011.59.57%20PM.png?dl=0

This topic is now closed to further replies.
  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Posts

    • Just another reason (aside from many others) not to use Edge. Firefox 153.0b5 DEx64 has a similar feature added recently in prior builds that I will turn off at some point when I get around to it. It's the new "Something looks suspicious" page that pops up here and there. It cleverly hides itself between web pages that I've actually visited; as a result, you know, of selecting a web page and telling the browser where to go. The interesting thing is that it does not produce these warnings from pages that I, as the only intelligent user of the browser in my system, have ever directed the browser to open! What seems to be happening is that the browser looks at all the goofy ad links on a web page I do actually open and selects one that "looks suspicious" and then creates the "something looks suspicious" web page, which is neatly inserted, as mentioned, between web pages my RB ("real brain") has directed the browser to load in a session. The thing is, I usually look at links I am considering to follow before I ask the browser to load them, and in cases I have noticed where the link does indeed look suspicious, most of the time I will choose to not follow the link at all. Doesn't everyone do this or something similar? I am picky about what I voluntarily load... (I don't like links that start off fine, with a site designaiton that seems normal enough but then is followed by indecipherable alphanumeric strings many, many lines long, etc. I tend to reject those because they look suspicious. They may not be, but I don't care... I'll stay with Firefox, of course, if for no other reason than they usually let you turn off the junk you don't like. And because it isn't Edge... But at some point Microsoft will come to realize that putting your bookmarks on the left side is a Good Thing for a lot of people, just as Microsoft discovered when it had the bright idea of nailing the Windows taskbar to the bottom of the screen, when for decades Microsoft browsers had left that placement up to the user. They have finally reversed the obscenity of that decision. Finally.
    • Google was using the old CATPCHAs data to train their LLMs. What is the say they won't use this camera data of users to train their LLM? these companies need some strict regulations!
    • Depends on what you need. Might be a bit clearer on what you plan to do with it. Sort of a waste if you get the newest and greatest, but don't know how to use it.
    • NTLite 2026.06.11200 by Razvan Serea NTLite is a Windows configuration tool that allows you to modify your existing Windows install or an image yet to be deployed, remove Windows components, configure and integrate, speed up the Windows deployment process. Reduce Windows footprint on your RAM and storage drive memory. Remove components of your choice, guarded by compatibility safety mechanisms, which speed up finding that sweet spot. Windows Unattended feature support, providing many commonly used options on a single page for easy setup. Easily integrate a single or multiple drivers, update or language packages. Package integration features smart sorting, enabling you to seamlessly add packages for integration and the tool will apply them in the appropriate order, keeping hotfix compatibility in check. One of the important new features of NTLite (compared to its predecessors) is the ability to modify an already installed the operating system, by removing unnecessary components. Supports Windows 11, 10, 8.1 and 7, x86 and x64, live and image. Server editions of the same versions, excluding support for component removals and feature configuration. ARM64 image support in the alpha stage. Does not support Checked/Debug, Embedded, IoT editions, nor Vista or XP. NTLite 2026.06.11200 changelog: New Secure Boot Migration support: Verification, certificate staging, and boot-manager/sector update across the Image, Updates, Apply, and Create-ISO pages (2023 CA migration, optional 2011 revocation, Anti-rollback, Boot sector choice etc) Secure Boot Host Readiness: Live host Secure Boot migration monitor and Servicing-task control Option under Image page - C:\Windows row, or load the host as the target - Updates - Secure Boot Image: 'Sort mounted images first' option for the image list in Menu-Settings UI: Hover description card for Components and Unattended pages, selectable text and quick access to Compatibility options Command line: Relay commands into the already-running instance Enables controlling already running NTLite via ntlite.exe Use /NewInstance to launch an additional instance using CLI operations (premium) UI: 'New instance' option via main menu instead of a secondary ntlite.exe prompt Apply: Hide individual Apply-page notes with a per-note dismiss (X), critical excluded Settings: 'Unsigned RDP file launch warnings' tweak (RDP client), bypassing the April 2026 security-update prompt on RDP connections Upgrade Image: Live OS and deployed image editing now unlocked on free/test licenses, same licensing as images Image: 'Recompress' option in manual dialog Remove Editions to shrink the WIM in one session Image: SWM part size set inline on the Apply page and image dialogs, split-size popup retired Image: Relative 'Last change' dates; editions grouped by build time to reduce noise Image: 'Forget - Missing' on the Edit-cache menu to mass drop entries whose folder is gone Components: Root groups reorganized - user-facing groups first, system/critical last Components: Show filter options to view components by Template or App-type, since Apps are now merged into groups Presets: Delete confirmation now lists the multi-selected preset names UI: Design update propagated to the rest of the tool UI: Filter and search match words in any order and partially, better results filtering Components Unattended: Input-locale language derives from the user locale, with an independent keyboard picker, enables combinations previously unavailable Unattended: Input-locale now allows for a user value override Unattended: Localization OOBE WinPE now can be copied with the new WinPE Copy OOBE localization toggle, enter locale settings once for both stages Updates: Downloader greys and locks updates the image already carries (hotfix and MSIX) Updates: Resume interrupted update downloads Command line: Many upgrades, see /?, now prints help to the console or redirected output UI-Translation: Finnish language added, also thanks for Chinese Traditional (Matt), French (tistou77), Italian (clarensio), Russian (RDS), Swedish (1FF), Vietnamese (Vu Anh Vu) Fix Components: Containers removal breaking Apps deployment Components: Microsoft Account had leftovers when Easy Migrate is kept Image: Export to an existing WIM improvements, Append renamed to Merge Image: Improved 26H1 live removal support Image: No more 'X:\ not accessible' popup for certain drives during image scan Presets: Manual image refresh picks up presets added/removed outside the app Tweaks: Disabled visual-effect animations no longer return after first logon on a new profile Tweaks: Live Visual Effects toggles (animations, drag full windows, font smoothing) now apply correctly Download: NTLite 2026.06.11200 | 20.5 MB (Free, paid upgrade available) Link: NTLite Home Page | NTLite Features | Screenshot Get alerted to all of our Software updates on Twitter at @NeowinSoftware
    • Ah. La Fontana De Incontinentia ! Bella ! Bella !
  • Recent Achievements

    • One Year In
      BA the Curmudgeon earned a badge
      One Year In
    • Conversation Starter
      rosiecharles earned a badge
      Conversation Starter
    • First Post
      KMilenkoski1202 earned a badge
      First Post
    • First Post
      carols23 earned a badge
      First Post
    • One Month Later
      Tom Willson earned a badge
      One Month Later
  • Popular Contributors

    1. 1
      +primortal
      504
    2. 2
      +Edouard
      257
    3. 3
      PsYcHoKiLLa
      151
    4. 4
      Steven P.
      93
    5. 5
      macoman
      67
  • Tell a friend

    Love Neowin? Tell a friend!