Why is my imac shutting down by itself?

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?

For my case, finally found out that it is the main board problem, I have changed the main board and the problem solved.

I have exactly the same issue as this except I unplug the power sully then replug and it will only then reboot. happens when using anything or nothing on the imac. What a pain in the arse.

Absolutely the same problem, but I do not have Apple Care and there is no way to pay for their experiments ....