Why is my imac shutting down by itself?

Annnnd...my machine is just flat blacking out again (turning off, to be clear). Not as often, but way too often. It always requires pulling the power cord for 20 seconds or so (presumably resetting SMC) to boot again; will never boot without doing that after a crash. A.C. has given in (and I don't blame him); I'm not asking for or expecting a response.

I guess I'm not ready to be fully convinced it's solely hardware. Hardware test shows nothing, I don’t get errors with the drive, and the temperatures never appear to exceed maybe 120 degrees (F).

So I’m wondering how many of you happen to use any of the following non-Apple apps, just out of curiosity: Xmarks, any Adobe Creative Cloud and its multiple processes, Canon EOS apps, Avast, Dropbox, OneDrive, SuperDuper, Canon MF (printer/scanner), LeapFrog (kids toy updater), or Wacom tablet apps. There are many more, but these are things that are at least always running something in the background on this machine. If there's some correlation, you know, I'd sure like to know it…the temperatures as I said aren’t bad at all, particularly with smcFanControl cranking away, so perhaps there's just some app or drive taking the whole thing down. Yes, I'm grasping now, but I'm pretty frustrated and would love to solve this thing.

Dropbox and OneDrive - I anticipate a crash whenever I use these apps and i am usually not disappointed, but if I keep the activity low, I usually make it. Same goes for mail - Apple Mail and Outlook. That's why I originally wondered if it had to with online activity before I decided heat was the issue, again before I decided I didn't know *** is wrong with this machine. Photos is a guaranteed crash.

I am planning on turning this into a Windows box when I replace it. It is perfectly stable in Windows.

In my grasping at straws I put Sierra on this iMac, a few days ago. Figured I have little to lose, though I was concerned that it would stay 'on' long enough to complete the update. It did, no problem. Two observations:

-Temps generally creep higher;

-I have not had one single shutdown/blackout since installing - this going from guaranteed every 20 minutes if running anything intensive. I've run my most angry apps, heavy lifting Adobe stuff, kept Photos, iTunes and Mail open, the whole thing, everything chugging away, rendered a decent sized movie...and it hasn't blinked.

Maybe coincidence. Or not. But I'm a whole lot happier, and hoping it lasts.

Hello All, I've been watching this thread intently. Late 2012 Mac 27 inch, Nvidia card,Geforce GTX 680MX, 3.4 i7, 16GB RAM, 1TB Drive. Started experiencing frequent random shutdowns about a month ago. Tried to do the basics to fix it, as far as installing all updates, PRAM reset, Reinstalling OSX, etc... Nothing worked. I finally brought it back to Apple, out of warranty. At the Genius bar, a Power Supply and Logic board were allocated. At which point i was not sure if i wanted to proceed with the logic board if that was the issue. They had the machine for 5 days for extensive testing. They found a fault with one of the 8 GB DDR memory chips. It should be noted that the memory fault was NOT found in standard testing. I received the machine back today. I will keep my post updated and report back within a week or so to see if the issue is resolved. The machine does seem a bit snappier at the moment. The genius bar did NOT reinstall OSX, and only replaced the memory chip.

i am right in the middle of this problem. started about a month ago. first shut down after a few blinks from the screen, which had begun about a week before. i restarted and was fine for a week. blinking gone but i made sure i backed up my stuff. then a few days ago it shut down again. if i uplugged it started up ok... eventually not. i called and we ran through some things when the computer was on, but kept going off. cleared cache. did a number of starting while holding command shift and other things. eventually would not start again even after unplugging. so in recovery mode apple had me reinstall the os. been on now for about hour and a half without shutting down itself. i even did a couple of shut downs myself with easy restart. no probs yet. they said to play with it hard for a while... so i have had a movie playing, while playing music, while surging internet.. and so far it is still up. i am concerned about working on an important project or attaching my external in case it shuts down again. maybe tomorrow. i too will post after i either have another problem or none for a while. since it has been a few days for you if you have had no more problems after all the hardware fixes.

Yes - same here. I can only get it back up if I pull the power cord and wait about 15 seconds before plugging back in.

Can you update graphic drivers on your own without taking it to an Apple store? I will definitely update to Sierra next. Some people have commented they were on Sierra when this happened though...

Hi,

It is an easy update. Go to system preference, Nvidia GPU is listed there, below the regular icons, click the icon for update.

Rajiv

Why didn't a "genius" from the Apple store suggest that in one of my 5 trips?😕

Did updating the NVIDIA solve the issue for you?