IMac CPU fan on full speed from start up

Hi,
For about 3 months now I've had a problem with my iMac where the CPU fan runs at full speed from start up, until I turn it off. It is extremley noisy and frustating.
I am pulling my hair out trying to fix it.
So far I have reset the SMC and PRAM and downloaded fan controllers. I've also opened it up and checked the sensors and cleaned out all of the dust, but this annoying problem still persists.
Is there anything else I could try that could fix this problem?
Thank you very much.

Thank you
I have tried to run the AHT many times, but for some reason I can't get it to work. I even tried booting it from the insitallation disk which someone else suggested.
I'm not sure where to go from here

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CPU fan running full speed after 15 minutes

My 3 month old iMac 24" has just started having fan problems.
Before I went away on a training course, it was working fine. Left it switched off at the outlet for 3 days, now the CPU fan runs full speed after the iMac has been switched on for 15 to 20 minutes.
CPU temp is 24C, CPU heatsink is 28C. Ambient is 17C. CPU fan is 3600rpm. Other fans are normal.
CPU usage is less that 10%. Only Safari is running.
I've tried resetting the SMC - left all plugs disconnected for 15 minutes, reconnected mains power for 15 minutes, reconnected USB, then switched on.
Still the same - OK for 15 minutes, then CPU fan runs full speed.
Anything else that I can try?

I tried again, following your instructions exactly. It seemed to work, running normally for about an hour.
I left all the USB devices unplugged, except for the keyboard and mouse.
I then shut the iMac down, but left the mains power turned on at the wall. When I switched it on again 2 hours later, all was OK for 15 minutes, then the CPU fan ran at full speed.
Shutting down for a few seconds fixes it for another 15 minutes.
I'm wondering if it's a software problem, although I haven't loaded any apps recently, apart from a few monitoring programs since this problem appeared.
Last night I left it running the extended hardware test for a couple of hours, and the fans were fine.
One thing that I noticed yesterday was that the "GPU" temperature readings in iStat were all missing (just "-" displayed) for a couple of seconds when I clicked on Dashboard. Is there a way to keep iStat open while I'm using the iMac?
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Fan runs full speed at start up

HP Pavilion p6-2037cb purchased 1/19/12. Windows 7. There is no error message.
When I turn the computer on after it has been off all night the fan runs full speed. This happened a few times in the first month. The first time it stopped itself after a few minutes. Then I learned to stop it by simply restarting the computer. This still works, but what used to happen a couple times a month, has happened twice in two days so I worry the problem is getting worse.
It never happens when I am using the computer because I do nothing that requires a lot of cpu's or causes heating. I just surf the web and do email. No games or videos or TV.
If it is a fan connection problem I would like to know the nearest repair service. I would rather bring it in than ship it. But I can't find a list of authorized warranty repair service centers. I followed links on HP Support Assistant but got sent to a site that does not list Pavilions.
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This question was solved.
View Solution.

Hello mpw101,
Seems like there was some confusion on the products your received and how it was explained. I have forwarded your thread to the appropriate team for review. Someone should be in contact with you shortly so you can discuss your HP customer care experience.
If I have helped you in any way click the Kudos button to say Thanks.
The community works together, click Accept as Solution on the post that solves your issue for other members of the community to benefit from the solution.
- Friendship is magical.

Mac Fan runs at full Speed from start, even in standby mode!

Hi Folks
Since this night my iMac's fan (summer 2012 model) runs at full speed from the beginning of powering it on. It even runs at full speed when I put the iMac to sleep!
So far I tried to reset the SMC three times, noo effect at all. I checked also the activity log, no processes or other activities are causing this. I read about defect HDD sensors, could that be the problem?
Any ideas how to fix or at least control this issue???
Thank you all for your help!
Matt
PS: That's a pretty annoying sound I'm hearing here, I feel like on the roof top of the Hancock Building at wind speed 200!

I have tried the SMC reset as instructed and the fan is running normally. It is a bit of an intermittent problem so I will monitor the performance of the computer over the coming weeks. If it persists, I will take it in to a service centre while it remains under warranty.
I guess the "cure" I had stumbled across by accident was nothing to do with turning the computer upside down, it was because I was unknowingly performing an SMC reset. The reason I turned the computer upside down was to find the serial number on the base of the stand so I could report the fault to Apple. Of course, when I plugged it back in to contact the support line, it worked fine.
Once again, thanks for your help.

IMac CPU fan speed at full blast.

I have a 20" iMac (A1224) and when you srtat it, the CPU fan goes full blast and never stops.
SMCFanControl reports the CPU fan at 3900 RPMs (full blast) but what is it trying to cool? The temperature is 27 degrees celsius (80° F), Activity Monitor says 93% of my CPU is idle. I've done all the normal SMC/PRAM stuff and it seems that my CPU temps is just fine. Something needs to be replaced.
Sensor
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DC Power Supply/SATA/Inverter cable
Also, yesterday it repeatedly shut down and restarted itself over and over and over until I unplugged it.

I realize that Eric. However, I am currently unable to: https://discussions.apple.com/message/21827955#21827955

IMac fans running full speed constantly.

A few days ago my iMac's fans started running at full speed, which they very rarely do. Now every time I turn on my computer after about a minute the fans reach full speed, I'm only opening Google Chrome, Twitter, iTunes and Coversutra which has never caused the fans to run before.
I've downloaded a program to monitor temperatures of various parts of the iMac but nothing is getting hot enough to cause the fans to run at full speed.
I have reset the SMC and PRAM multiple times which hasn't fixed the issue. I've run the Apple Hardware Test on the original install disks which gave an issue number relating to the fact that the fans are running on full speed.. surprise surprise...
The computer isn't very old, was purchased almost to this date in 2009. It's an 8,1 iMac with a 20" screen. 4GB RAM and a 2.4Ghz processor.
Would be extremely appreciative if anyone has any idea how to fix this problem.

Get free iStat Pro. This code is obviously related to the HD, but I don't know if it necessarily means the HD is failing. It will show the fan rpms to see which fan is revving up. If it's only the HD fan that is climbing up, this may be related to the HD temperature sensor being faulty. Thing is, the sensor is probably integrated into the drive, so if it's only that that's failing, it will still involve replacing the drive.
http://www.islayer.com/apps/istatpro/
Also, run the demo of SMART utility to check out your drive.
http://www.volitans-software.com/smart_utility.php
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Hi,
The processor usage is relatively low (like 10%) but the fans are full speed at all times from the second the power comes on. I imagine there isa problem with the mother board? I've tried resetting all the SMC stuff, no effect.
Any ideas?
Is this something Apple Stores will charge to fix?
Thanks

It could be either faulty, leaking caps, or a failing power supply unit. Yes, Apple will charge since you are long out of warranty. You can open the machine and check the caps yourself, they look like little soda cans, and their tops should be perfectly flat and there should be no exudate.
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I have a 24" aluminum iMac (iMac7,1) which had a hard drive failure. I replaced the hard drive myself and all went well. However, now my fans are running full speed. This appears to be because of a problem with detecting my temperature sensors now. During the install I only touched two temp. sensors, the hard drive sensor and the LCD sensor. I made special care to make sure I got the sensors back in place.
Using iStat Pro the only temp. sensors I have readings for are the Hard Drive and Power Supply. I am missing ambient, optical drive, CPU, and display. Out of those the only one out of the missing ones I touched are the display one. I have since gone back into the machine to ensure the LCD temp. sensor was fully connected and to ensure the HD one was connected (although that one is working).
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Mechanic man thanks for the response.
Besides for the fan noise the machine is running like a champ. The new hard drive seems to be fine, actually my machine boots up faster now than it ever did (faster drive I suppose). I ran disk utility and did a verify on the new disk and it didn't detect any problems.
The fans run at full speed immediately at power-on (maybe a one second pause). I first noticed it when I booted up from the startup disk to do a time machine restore. So yes, it occurs when booting from a system disk.
I have been unable to run Apple Hardware Test, I saw this step in some google searching and Apple docs but I don't seem to have AHT on any disk. I have tried putting the Snow Leopard disk in and holding "d" at boot time to no affect. I did some further reading and found that AHT may only be included on disks that come with the machine. I am going to try to dig up my Tiger disks and see if I can get AHT to run from there (10.4 was the factory installed OS when I bought my iMac).
I am also considering trying to re-install SMC firmware (http://support.apple.com/kb/HT2368). Even though it will be the same version at least I know the SMC reset has taken place.
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Thermograph data suggests the fans are behaving appropriately, so I think the system is running hotter than in the past. Anyone have other ideas? Anyone else seeing this classic behavior?

Have a look at one post I made some months ago :
http://discussions.apple.com/message.jspa?messageID=1621311#1621311
I discovered that going back to Classic from one user to the other make a large use of CPU (80 to 90%) thus rising the temperature and conversly the fan...
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Hope this helps.
JfS

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Tecra A10-S3501 Power Light Flashes, Fan Runs Full Speed.

Greets all,
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IMac fan runs full speed on shut down

Hi Everyone, this has probably been discussed before, but I may have some new information to add that could interest the tech savvy people out there.
My iMac runs perfectly fine; more or less as it did when it was new. In OS X, when I shut down the computer, the fan runs at full speed indefinitely, unless I unplug it or boot back up. When running, the fan is perfectly normal (increases speed a bit when doing something taxing). I've never heard it at full speed before until this started happening. After following the SMC reset procedure with no luck (I'd like to add that the SMC reset only says it relates to fan problems when the computer is ON), I searched a bit and noticed there are one or two cases where a problem like mine was believed to be a motherboard issue, and therefore a costly fix.
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So then, my questions are these:
-What is the difference between shutting down from OS X and shutting down from Windows?
-Is an operating system allowed/able to touch BIOS settings?
-Is it possible to still be a hardware problem, given this difference?
My default OS is OS X, and it makes no difference if I change that to Windows. I also did a PRAM reset, even though that has nothing to do with fans...
I'd certainly like to avoid a costly repair, if it's just something that a future software update will fix.
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Hey, thanks for the response.
The "Startup after power failure" option was not checked.
I'll try the Extended Hardware test when I get some spare time again - probably tomorrow night. After that I suppose it is time to either take it in for service/diagnosis, or just keep using Windows to shut it down. Sleep mode is unfortunately not an option because it eventually goes into hibernate state, which has the same problem as turning off. I don't like to leave the computer on all night, just from a power-conscious point of view (but yes, I know today's iMacs use very little power). I have it on a power bar now, so I just flick the switch off when I power down. It's not bad as a temporary solution - perhaps permanent if the shop says they have to replace the motherboard.
As for contacting Apple... I had a 1-hour conversation with them about my iCloud account, because it suddenly stopped working on this Forum site (hence the new account with no points). Anytime I sign in, it forwards me to the "page not found" error page. This happens even if I sign in on the apple store then click over to here. If my login info has an open session, then Discussions just doesn't work. They were completely baffled (understandably), because the account works on iTunes, iCloud.com, Apple store, and everywhere except Discussions. I was on the phone with them for over an hour - and the kicker is, even after telling them the same thing happens in other browsers, from other computers, they actually forwarded me to the iMac team. This is when I lost faith in Apple support. When one of the staff asked why I don't just phone about the issue, instead of trying to get access to the forums just to post about it, I told them I didn't want to pay $50/hr (or whatever the insane rate is for Mac hardware issues), only to be put on hold and sent around between departments, just to be told I need to send it in for service, which I'm probably not going to do if it's the motherboard. They never did get back to me about my account issue...
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Hi , Thank you for your query. I understand that you had the display replaced by HP as it quit due to a hinge issue. When you got it back after they had done full service - cleaned it up, updated bios, replaced fan. The cooling fan was replaced as they claimed it was noisy, but to be honest you had never noticed it. As soon as you took it home and powered it on the fan went crazy,running at full speed non stop. The speed sometimes would change from full to nothing every 2-5 minutes. After a few hours it seemed to settle down and work normally.Occasionally the issue would return for a hour but then return to normal.A couple of days ago the issue returned but now remains constant. Here is a link to HP Notebook PCs - Fan is Noisy and Spins Constantly, PC is Warmer than Normal. As this did start just after the repair, I would contact HP support, explain the issues and get your options if the above has not helped.Please contact HP support at Contact HP Support. To say thanks for replying please click the thumbs up icon below.If this fixed your issue, be sure to let people know by choosing solution provided. Best of Luck!!

IMac CPU fan on full speed from start up

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