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My iMac 27" from 2011 has recently been experiencing random freezes. It either restarts or I am forced to reboot (holding power button several seconds). When frozen the iMac does not respond to pings and nothing at all is written to the system logs.

Symptoms are graphics issues which happens just before it freezes. Tearing, flickering and random graphical artifacts. When it freezes, it usually shows a screen with striped patterns, a grey or black screen. I have checked CPU and GPU temperature and there are no problems with overheating.

Once it hangs and I reboot, the computer usually fails to boot and hangs during boot with a gray screen. At this point I cannot get it to boot at all, except into single (console) mode. Checking disks reveals nothing and running apples hardware tests tells me everything is ok.

If I leave the computer turned off for a while (say over the night) and then try to boot it, everything works.

Someone suggested it may be power-related somehow, but I have no idea how to check that.

Any ideas how to diagnose this?

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Are the stripes pink and/or green? – duci9y Dec 23 '12 at 8:49
Black/white mostly. I've seen red squares as well. But green sometimes, yes. – Martin Wickman Dec 23 '12 at 9:18
Whenever I’ve had graphics problems (Usually overheating) whether or Mac or Windows, I got green and pink stripes on my screen. Don’t focus on graphics failure too much. – duci9y Dec 24 '12 at 7:47

2 Answers 2

up vote 0 down vote accepted

Turned out to be a faulty graphics card. I replaced it with a new, and problems went away.

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I have seen the same problem on an iMac 27" 2011 with an i7 3.4Ghz. Apple has a replacement program. You can find more infos here:

I found a temporary solution. The iMac can recover from this mode. Just put the iMac to sleep. When it wakes up it's fine again. Use the sleep button on your Mac. Alternatively you can do this with SSH. So you could use your phone to fix the iMac. Just connect to your Mac with SSH. Then type the following:

pmset sleepnow

After that you should disconnect. If your machine doesn't sleep. You could use or a similar application to make sure it goes to sleep.

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