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We have a late 2012 27 inch iMac. It has a 2.9 GHz i5 processor and a 1 TB fusion drive along with 8 GB of RAM. I recently upgraded it to El Capitan.

A couple of months ago it randomly started to not wake up after being asleep. We would have to unplug it and plug it back in to get it back on. It also would just randomly shut off while we were using it. If we had music playing, the screen would go black and the music would stop so it's not a display issue.

I took it in to our local Apple store and talked to them about it and they said the first thing they would do would be to reinstall OS X. So I decided to do that myself but that still did not fix the issue.

I then lugged it back into the Apple store about 2 weeks ago and left it with them for a few days to get to the bottom of it. They repaired OS X and then put it through a battery of tests to see if there were any issues with memory or the hard drive but everything came back fine. They then left it plugged in with a bunch of graphics programs and videos playing and still had no issues.

At this point, I was thinking it was a power issue with the plug we were using. When I brought it home, we left it plugged in and running in our dining room for a few days and did not have any issues. I gradually loaded programs back on and still did not have any issues. I then moved it back upstairs but the other day it started acting up again on my wife. I moved it back to our dining room thinking it was the outlet up there, but it's exhibiting the same behavior there now.

After doing some searching online, I found this post http://samwize.com/2013/10/13/imac-shutdown-suddenly-and-randomly/. It seems to describe the issue we are having.

I was able to check the Console and it says the cause for shutdown is 0. I'm pretty sure the Apple store ran the hardware test and memtest while it was there. I have repeatedly reset the SMC, which the Apple store employee informed me is basically what I'm doing every time I have to unplug it and plug it back in.

The only thing I haven't done yet is a PRAM reset.

So, now I'm not sure what to do next. According to that post, his logic board needed to be replaced. The Apple store quoted me $583.32 to replace the logic board if that was what was needed. But I only want to do that if I know definitively that's the issue.

So sorry for the long post, but does anyone have a suggestion on how to root cause this?

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    did you ever get an answer to this? We have the same exact model and the same exact problem. I could have written this post: we also tried different outlets, left it with the Apple people for a few days, and no issues were found, but now we are finding it is shut down every time we try to use the computer (progressively getting worse). Would love to hear if you resolved the issue. The only thing left that I can think for us to try is to replace the power cord... Thanks for any follow up on this. – user169119 Feb 6 '16 at 1:10
  • No, nothing definitive. We have been shutting it down when we aren't using it and that seems to help. I think we are finally going to bite the bullet and just pay to have the logic board replaced and hope that fixes it. If it does, I'll update and let you know. – njkroes Feb 9 '16 at 16:51
  • So we got a new power cord and the issue still persisted. We decided to bring it to a local Simply Mac store to see if they could diagnose it. They did not see it shut off, but when they shut it down, it would not power back on without doing the SMC reset. With some additional testing they narrowed it down to either RAM or a bad logic board. They replaced the RAM with known good RAM and the problem still persisted. So now we feel pretty confident that it is the logic board. We are going to replace it next month and I'll update on whether that fixes the issue for good. – njkroes Feb 26 '16 at 16:16
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    Probably not what you're hoping for, but have you tried putting a UPS between the Mac and the outlet you're using? It's possible that one outlet has some slightly faulty wiring, for whatever reason, and it leaves a small static charge in the Mac's PSU if/when it mildly under- or over-volts (it's also possible the problem is inside the Mac itself). Grab a $50 UPS from Amazon or Best Buy or wherever and give it a shot. – agentroadkill Mar 5 '16 at 18:48
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+25

Chances are this is a bad power supply board.

enter image description here

Shutdown error codes can be negative or positive values; negative being hardware and positive being software.

A "0" code is right there in the middle where it usually means a sudden loss of power. See this post for more info: Are OS X shutdown cause and sleep cause numbers listed/explained anywhere?

If any part of your logic board was beginning to fail, it would generate kernel panics which would be in the log and likewise, if you had software causing issues, there would be a log of a crash somewhere.

I've see this previously on other platforms whether it was a Mac, or a PC, or even enterprise grade servers. What is probably happening is that a component in your power supply is getting too hot and consequently shutting down. If you take it to the Apple store, they probably have the ambient temperature much lower than you home so it never has a chance to over heat. Just when you think everything is good, the symptoms return.

Is your downstairs location warmer or cooler than the upstairs location where it normally resides?

If you need to replace your power supply, rest assured these are fairly easy to come by and fairly inexpensive. At the time of this post, they were trending around $65. It's also very easy to replace (see the guide at ifixit.com)

While you are in there, I would swap out your (spinning) HDD with a SSD. I just installed a Samsung 850 EVO SSD in my 27" iMac and it is substantially quieter and cooler than before.

Disclaimer: There still is a chance it's the logic board, but it's highly unlikely. Before you do any expensive logic board replacements, try replacing the power supply first.

One more thing....I would definately follow agentroadkill's advice from his comment and put a UPS (battery backup) between the outlet and your Mac.

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    Interesting. Thanks for the info. I was thinking that the visit to Simply Mac that I mentioned in my above comment was definitive in it being the logic board. Based on the feedback they gave, you still think it could be the power supply? Just curious. Oh and we ran into this issue downstairs as well as upstairs. We usually keep our house between 69 degrees in the winter and 74 in the summer. – njkroes Mar 10 '16 at 15:08
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    The question is, how did they determine it was the logic board when they did their diagnostics, they found no issues (para 4, last sentence)? To me, that's just a WAG on their part. There are only 2 ways to determine if in fact the logic board is at fault is to a) diagnose at the component level and b) swap it into another iMac with known good components. Ask them - what SPECIFICALLY on the logic board failed because what they are saying is the automotive equivalent of "we must replace your engine even though we never saw your car shut off." – Allan Mar 10 '16 at 16:08
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    Keep me updated - I will try to assist wherever I can. As for the UPS, you can't go wrong with that purchase. It will act as a power conditioner and protect you if/when the power goes out. Cheap insurance if you ask me. – Allan Mar 15 '16 at 18:04
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    I believe I can definitely say that the power supply was the culprit. I was not comfortable replacing it myself so I dropped it off at our local Apple store last week and had them replace the power supply. I picked it up on Monday and we have been using it this week without any issues. It has remained on every night. Thanks, Allan! – njkroes Mar 24 '16 at 23:39
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    @Dave I don't remember. Feel like it was $100 to $200 though – njkroes May 23 '18 at 18:38
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i had the same issue with my iMac 2010 edition, i read that apparently my hard drive was failing and thought why not give it a shot and replace it, once i did the problem did stop. However in your case it may be a bit more difficult to replace the hard drive seeing as you have a slim model.

if you get the chance, try this CNET Check if your hard drive is failing

  • If you have a failing hard drive, you should verify that by checking the SMART status of the drive. DriveDX (free version) does a very good job at this. Changing the drive in the 2012 iMac is much easier than you would think - once you get through the adhesive, it's as easy as the 2010 model. – Allan Mar 11 '16 at 11:41

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