Late 2012 iMac 27" turns itself off

[bomaya]

During my second year of owning a late 2012 iMac 27" I found it started turning itself off. Unfortunately I didn't have any AppleCare so just put up with it. Since the new year though I've had it in with a local computer shop twice to see if they could solve the issue.

I'd googled the issue and found several people with the same problem but multiple different answers. Common one was overheating so I tried the MacFan app but even set to maximum that made no difference. First time in the shop, they found the insides and fan caked with dust so cleaned it all out and tested it, seemed fine. Trouble is the fault is very intermittent - it can go off after one hour or work fine nearly non-stop for days.

Second time, they thought it was definitely the power supply so got a new one from Apple and fitted it. Again, seemed fine but a few days later it went again. They want to test it again but I'm starting to wonder if it will ever be fixed.

Every time it happens, I have to unplug the plug, leave it for 5 minutes then plug in and turn on. If I don't do this, it will not re-start. it doesn't seem to matter if I have multiple things running or nothing. It can be after one hour or eight.

Do any of you have experience with this issue or any solutions or answers?

 

[rigormortis]

anything power related might be the fault of the smc controller

instead of waiting 5 minutes, does unplugging it and holding down the power button for 5 to 30 seconds allow the computer to come on again?
( you have to unplug the computer and wait for any residual energy of the capacitors to dissipate to allow the smc controllers' ram to erase )

if it is the smc, then 5 minutes of it being unplugged is the same thing as unplugging everything and holding the power button on for 30 seconds, and then tuning it on


or

maybe its the cpu overheating...

the genius bar triage program would tell you if any of the computers sensors is not working, it would be more reliable test then your fan test

if it needs 5 minutes and the power button trick won't work. its gotta be something related to the capacitors . maybe its the capacitors on the lcd screen itself.

 

[bomaya]

anything power related might be the fault of the smc controller

instead of waiting 5 minutes, does unplugging it and holding down the power button for 5 to 30 seconds allow the computer to come on again?
( you have to unplug the computer and wait for any residual energy of the capacitors to dissipate to allow the smc controllers' ram to erase )

or

maybe its the cpu overheating...

the genius bar triage program would tell you if any of the computers sensors is not working, it would be more reliable test then your fan test

No, I always have to wait. Originally I was unplugging, holding power button in for 30 seconds, then plugging in, waiting another 30 seconds. Now I skip the middle step but leave it a few minutes as the power button bit makes no difference it seems.

Main reason I haven't tried a real Apple shop is my nearest ones are all in middle of busy shopping settings and lugging a 27" beast around isn't that easy :/

 

[rigormortis]

i edited my post

I'm thinking you got bad capacitors
[doublepost=1460387110][/doublepost]or a bad heat sink

 

[boto]

I had this very issue for over a year now and I own the exact model. I tried going to Apple to resolve this multiple times, replacing my HD, power supply, or checking on other hardware and they couldn't seem to reproduce it. Every time I returned home with the machine it would still shut off randomly after a few days and like you mentioned, required unplugging and waiting for a few minutes for it to turn back on. However, the longer I left it unplugged determined how long the machine would remain on, perhaps overheating.

Good news, though, it has been over a month since I experienced any random shut downs. I found that reinstalling OSX, updating to the latest version, and enabling 'Start up automatically after a power failure' did the trick for me. I'm not sure why this works, but after purchasing a UPS, cleaning the back of my machine on a weekly basis, and turning it off whenever I'm out resolved it. I no longer have to worry about losing my data and documents.

On a side note, it might possibly be Chrome glitching and overheating before and my guess is the latest version is stable. When this was happening the only application open was Chrome for me and sometimes the sudden shuts downs occurred while watching some videos.

 

[bomaya]

Good news, though, it has been over a month since I experienced any random shut downs. I found that reinstalling OSX, updating to the latest version, and enabling 'Start up automatically after a power failure' did the trick for me. I'm not sure why this works, but after purchasing a UPS, cleaning the back of my machine on a weekly basis, and turning it off whenever I'm out resolved it. I no longer have to worry about losing my data and documents.

On a side note, it might possibly be Chrome glitching and overheating before and my guess is the latest version is stable. When this was happening the only application open was Chrome for me and sometimes the sudden shuts downs occurred while watching some videos.

Thanks, I will try the leaving unplugged option as I had six shutdowns in six hours last night. Worst ever and that was after updating OSX.

Can't be Chrome for me as I don't use it.