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iMac keeps turning itself off - won't come back on again

I have an intel-based iMac that I've had for less that a year. Yesterday with no warning it turned itself off - the screen suddenly went black, as though someone had pulled the power cable out (which they hadn't, of course!) - and when I tried to turn it back on again it was unresponsive. I checked the electric sockets - they were fine.

Using my MacBook I went online and, following some advice I found there, reset the SMU. That seemed to work - the iMac started up again. But then twenty minutes later it 'died' for a second time. I left it for the afternoon, periodically seeing if I could turn it on using the power button. I couldn't. After about five hours I got it to start up again with no problems, long enough to run the Hardware Test. It found no problems. I restarted the computer - started up fine again. Then five minutes or so later died for a third time.

Following a suggestion in the manual, this morning I have reset the NVRAM. Once again, the iMac started up fine but died shortly afterwards.

Please, does anyone have any ideas? I haven't recently installed any new software or anything like that and I wasn't doing anything unusual when the problem first manifested. The cable is obviously OK, the wall sockets are fine. What else could it be?

iMac, Mac OS X (10.5.5)

Posted on Sep 30, 2008 2:35 AM

18 replies

alweston

Level 1

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Nov 10, 2008 6:41 AM in response to Bodomann

I'm getting a new power supply for the second time this year. Oops, I mean for the first time. I took the machine in for service this morning, and they're now saying that the power supply was never replaced even though I have paperwork that says it was.

Nov 10, 2008 1:52 PM in response to Celia78

Had the same issue recently with an iMac purchased in May. Went through a host of suggestions, but once I had my logic board and power supply changed, everything has worked again.
The tech who worked on my iMac was fairly sure it was the power supply, but installed the logic board as well since it has been an issue and is right next to the power supply.
As far as heat issues, I don't beleive that was the issue. To check, I have downloaded the iStat widget. Now I just hope that the power supply was a lemon and that nothing else was negatively impacting it.

Fortgal

Level 1

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Nov 17, 2008 5:45 PM in response to highcountry

Dear Highcountry and thread members, I did bring my iMac back to the Apple Store, where the genius team planned to replace the power supply, logic board and also found a problem with the sound! However, a few days later, they called to say I was getting a new replacement machine as there were so many things wrong. I never got the exact rundown, but I did pick up the new replacement iMac today. The store team transferred all my data, and I am hopeful that I will no longer experience the random shutdowns and endless restart loops with this one. Thanks to everyone in the thread for the support, and I urge all to be sure and get that extended warranty!

iMac keeps turning itself off - won't come back on again