Q: iMac 27" mid 2011 - Intermittent issue with CPU fan running at full speed and sleep mode.

reset the SMC

Reset the System Management Controller (SMC) on your Mac - Apple Support

4ntoine wrote:

 

Yeah I know, first thing that I tried. Not doing anything.

Sometimes it takes 2-3 attempts at resetting the SMC for it to work, please do this and post back.

Yes, if that does not work then you may have a hardware issue. In that case you should take it to an Apple Store or Apple Authorized Service Provider who can properly diagnose and repair it. If it is hardware it's likely a fan sensor or the fan itself, either repair is relatively inexpensive. Please don't ask me about prices, please ask the store or AASP.

Hmm, just thought of 3 other things you can try.

1. Restart the computer in Safe Mode, this will clear some caches. It is possible a cache has become corrupted. To restart in Safe Mode hold down the Shift key on restart until the computer fully restarts. Then restart normally and re-test.
2. If that does not help, disconnect any peripherals connected to the computer and retest. If the problem disappears then it's a process of elimination to determine which peripheral is causing the issue.
3. Finally turn off sleep and see if you can put just the display to sleep instead of the complete computer.

If you take it in to the Apple store for a free diagnostic be sure to have them check the GPU to see if it may be contributing.  Your model is covered by this replacement program:  iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support.  It's unlikely but worth having them check it out if you go in.