27 iMac (2012) crashes and requires smc reset to turn on

I have the same problem, 2012 iMac, 27" i5, 680mx, fusion drive.

I only got this problem after I moved house.

People have said it doesn't replicate when at the genius bar.

maybe poor power quality effects it. Voltage spikes, high line voltage etc.

In australia power is technically 230v but it can be +10/-6% of that legally. So maybe your local distribution quality is effecting the issue?

would also explain why replacing internals also doesn't always fix the problem.

Not saying its not a hardware issue because they should be able to handle anything within supply standard, but it could explain why some people cant replicate it elsewhere.

I thought that, too. Coincidentally I needed to have some electrical work done, so I asked the electrician to test the two outlets I had tried (different circuits). Both outlets fully conform to UK specs, so that should not be an issue.

That's one of the first things I considered when 2 Genius Bar could not duplicate the issue and back home the iMac keep shutting down itself.

But how would we explain that these iMacs work just fine plugged in the same place for a couple of years and suddenly start failing?

It is this inconsistency that makes the identification of the root cause quite difficult in this specific case.

It definitely isn't the outlet with mine as the shutdowns have occurred in multiple locations. Unfortunately none of these locations have been at Apple. The biggest difficulty on my end is that I live an hour and a half away from the nearest Apple Store, so every time I bring it in the problem doesn't surface. Further complicating matters is that the iMac passes all of the diagnostics. For me the issue only occurs when I'm editing in FCPX, and if the iMac has been unplugged for a while, like the drive to the Apple Store, the problem takes a while to resurface. But I still can't understand why when editing the problem always shows up eventually, but when running diagnostics it never surfaces. I think that if my iMac comes back from Apple for a second time without the problem surfacing I'm just going to request to edit on it at the Genius Bar until the problem shows up. I'm really out of options otherwise.

Thanks for this succinct update, I've been reading this entire thread after my late 2012 27" iMac started to turn off over the past 2 weeks.

I've noticed my iMac has a much higher frequency of shut down when playing computer games—and once it has shut itself off there first time into a gaming session, the next shut downs come much quicker. I've started playing games in a window so that I can still see the fan speed and temperature of my computer. Anywhere from 144–163 degrees F has triggered a crash, although 163 seems to be the peak of heat at any given time while playing and the game can run at the temperature for a whole match without the shut down.

After crashing during a game, my computer is more likely to crash doing something much more mundane, like opening Maps. But for the most part, I can be streaming from the internet for hours on end with no intermittent shut downs. It seems the "bout of shutdowns" is always started with a period of more intensive usage, like gaming.

Mine also crashes during games more frequently. originally I only noticed it during games but recently it's crashing at random on the desktop too.

OK Guys,

so it's just shutdown again after nearly a month of no problems....aagh!

No warning (as usual) and this was after being in at an Apple Authorised Reseller recently, who could not replicate the issue.

I was editing a few pics today in the Photos app and then it switched off.

I've just got off the phone with Apple Support who are collecting it hopefully in the next 2 days to take it away for repair. They seem to think it's an hardware issue but won't say what it is until they've had it in for repair.

I'll let you know how it goes.

Just received my iMac after the third trip to the Apple Store. Because the problem doesn't show up during diagnostics or stress tests their suggestion was to wipe the drive again, but for me to do fresh installs of all of my software, the first time they wiped my drive I restored from a back up. After doing fresh installs the problem surfaced once last night, and again after a few hours of editing today. And so it goes...

I've had mine back from the store now for a day and it hasn't shut down in that time. While it was there it had a power supply replacement. Looks promising but will update you all if anything else happens.

I'm beginning to think the power supply is the root of the problem. It seems to be the only thing that some people have had replaced where the problem didn't at least immediately resurface.

They seemed pretty convinced this was the case. The engineers pointed to the log showing that there had been a power interruption when it had switched off. They replaced it with a new 300W power supply which I assume it standard?

Did you run Apple Diagnostics to determine of it is hardware related?

https://support.apple.com/en-gb/HT202731

Yes but it showed nothing was wrong that it could detect.

If you don't mind me asking what do they charge for a replacement of the power supply? I still have applecare, but after six months of this problem, and considering I basically make my living off of this iMac, it's almost just worth it for me to pay out of pocket and move on if it will actually remove the issue.

For the third time in assistance in an Apple Store.
Now it seems they are going to change the Hard Disk and the power supply for a cost of € 385,42 but I am quite sure that this won't solve the problem of the shut down.

From Italy