2012 21.5" iMac - power issue

You could try a SMC reset, but I doubt that it will fix your problem.

Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve

Shut down the computer.
Unplug the computer's power cord.
Wait fifteen seconds.
Attach the computer's power cord.
Wait five seconds, then press the power button to turn on the computer.

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Mountain Lion seems to be particularly prone to problems in waking from sleep (just do a search of various forums and you will see how widespeead this is). If your iMac is the latest model (ie without any optical drive), then it might be worth setting it to never go to sleep. I have mine set this way and, after it has been idle for a little while, it stays very cold so I cannot see that it is using much power at all.

I too apologise a complaint is my first post!

However, I am having exactly the same problem. In Dec my new iMac arrived and after just over a week the computer went off and would not restart until unplugged. But then, after a few minutes would turn off again. As this was within 14 days of purchase I called Apple and they sent out a replacement...

All good right...

However, now I have had the replacement for just over 1 week... the issue has started again...

I am either very unlucky that I have been sent two dud computers, or there is something else going on, such as a fault with the design.

I had a old white iMac plugged into to same devices and same power sources and I never turned that off for about 6 months. No problems at all. So at the moment I think I can rule out an issue with power in my house.

Really frustrating that this has happened. Unlikely that they will replace the replacement, which means I need to try and get an appointment at the Apple Store to take it in... Seems like I will be without my computer for some time until this is sorted.

I am calling support tonight to speak to them and see what they will do. I will let you know if I get any information that might help others on here.

It only comes back on if I remove the cable, then hold down the power button on the back for 15 seconds then plug it back in. However, it only stays on for a few seconds before it goes off again... Absolute nightmare.

I also just want a working machine, was excited to get this new super thin iMac... but has been nothing but hassel since it arrived.

Yet another first post complaint:

I had the same problem - new 21.5 iMac worked fine for about 3 weeks then wouldn't wake up one morning. Got it to power up after pressing power switch multiple times. Worked all day, but then the problem repeated the following morning, and this time I could not get it to power up again. I took it to my local Genius Bar and was immediately offered a new machine in stock.

But once they learned it was a custom order, I was told that they would give me a gift card in the value of a new iMac with the same specs and that I should go home and order a new one. I wasn't very happy with this - Why should I have to go through the hassle of re-ordering a computer for what was essentially an exchange of a defective product? So the store manager took care of the re-order (and applied a $50 gift card to it as compensation for the hassle), but I have to wait 2-3 weeks for delivery.

I am pleased they offered a replacement, but annoyed at the long delay in getting it and at the convoluted and inefficient way they have of handling exchanges. I hope my replacement iMac arrives sooner than predicted and that this power issue isn't a recurring problem.

Apple is loosing money...with all those replacement ..maybe they have to be more carefully from now on