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0157988944 · Feb 24, 2008, 02:31 PM

My Aluminum iMac 20" 2 GHz, 2 GB RAM, is acting weird lately. All of a sudden, its fans will rev up to full speed and stay there. iStat Pro tells me it's the CPU fan, but the CPU is no hotter than anything else, between 30 and 35 degrees. The only parallel I've been able to draw between the different occurrences is that they've all happened immediately after I closed a Safari window. Also to note, if I put it to sleep and wake it back up immediately, the fans rev after about 20 seconds. If I put it to sleep and wait 10 minutes, the fans seem to be fine.

I'm not sure what triggered the first occurrence, as I was downstairs. I had put the iMac to sleep, and when I came back, it had woken up and it's fans were revving... really loud.

I'd rather not take it in to Apple because I don't know how to reproduce it and it seems to take as long as 6 hours of use to happen, so they wont be able to see it.

Any ideas? I've already reset the SMC.

seanc · Feb 24, 2008, 03:37 PM

Resetting the PRAM and SMC is the only thing I can suggest. Flash will hog CPU usage and make the fans run quickly, it could be flash or a runaway process.

0157988944 · Feb 24, 2008, 03:47 PM

If it happens again, I guess I'll try the PRAM> I think one of the fans is screwed up though, because there is nothing really abnormal about the CPU. No hogging processes, no high temps, nada.

JKT · Feb 25, 2008, 05:34 AM

If you use Activity Monitor, is there anything sucking up CPU cycles when this happens?

0157988944 · Feb 25, 2008, 07:30 AM

nope.

P · Feb 25, 2008, 08:33 AM

Try vacuuming the air vents. That can make a huge difference.

0157988944 · Feb 25, 2008, 11:33 PM

I'll try that out, I guess. nothing else seems to be working. Ugh.

0157988944 · Feb 26, 2008, 07:00 PM

Fixed it!

turtle777 · Feb 26, 2008, 07:12 PM

Wow, whodathunkit.

I gotta remember that.

-t

forumhound · Mar 16, 2008, 09:32 AM

new one out of the right out of the box did that. unplugged and rebooted and all seems normal now...

papfol · Aug 18, 2008, 03:21 PM

Originally Posted by adamfishercox

Fixed it!

I recently bought an iMac 20” and I have experienced the same behavior of the CPU fan. How did you fix it?

0157988944 · Aug 18, 2008, 03:37 PM

If you read the whole thread, it was vacuuming the air vents that did it.

papfol · Aug 18, 2008, 06:08 PM

Yes, I read the whole thread. But my iMac is less than a month old and has the same problem. Vacuuming the air vents did not work and room temperature is less than 20C.
Thanks.

schepers · Sep 30, 2008, 04:18 PM

I have the same fan problem with my iMac 20" which I purchased 6 months ago. Tried vacuuming with little luck. I even brought it into an Apple repairer, but it didn't want to do it for them??!! I am keen to learn if anyone else has any more information or fixes for this problem. I suspect there may be more cases out there!

JKT · Oct 4, 2008, 09:39 AM

Did you reset the SMC yet? To do so, shut down and physically disconnect the iMac from the power for at least 5 minutes.

schepers · Oct 7, 2008, 09:21 PM

Yes, did the SMC a number of times without any difference

schepers · Oct 27, 2008, 06:02 PM

For the latest update - I took my Imac in for repair. They replaced the logic board, which seemed to solve the issue. However, after a couple hours of use, I've got the same problem again. Looking at other forums, this is a big issue and I would love to get an anwer. Frustrated!

schepers · Nov 18, 2008, 10:17 PM

Another update - took the machine to the Apple repairers a total of three times. After replacing the logic board and the fan, they also replaced the power unit which was now showing up as having a problem in a hardware test. This seems to have fixed it. I noticed on some other forums that replacing the power unit has also worked for others. The tech. said that the power unit has a sensor on it which was malfunctioning.