Late 2012 (27") (i5) iMac - Keeps shutting down!

After 18 months of shutdown agony I was very frustrated. The local Apple Shop could never replicate the problem or find anything wrong with the computer.

In the middle of autumn here in southern Australia the problem became so bad that the computer was shutting down several times a day, including when it was asleep with no programs running or connection to the internet or to the Time Machine backup. It also used to sometimes die when I was shutting the machine down.

The straw that broke the camel's back was when it did that during a restart after a system update and corrupted the operating system so badly that Apple Support suggested I wipe the hard drive and reinstall the operating system from scratch. Even getting the existing information off the computer and onto a portable hard drive became a nightmare because the computer would abruptly shut down while I was doing it.

In desperation I decided to do what others have done and get the local Apple Shop to replace the power supply unit. Before replacing the power supply the local Apple Shop tested the computer again and still couldn't find anything wrong. One month later and I haven't had a shutdown.

If you are having this shutdown problem I would highly recommend that you try replacing the power supply first before spending money on replacing video cards, screens, CPUs etc.

You will probably have to insist that they do that first because they will still argue until the cows come home that it is likely to be one of the other components and not the power supply. Given how often replacing the power supply has solved people's shutdown problems shows that is not always true.

I hope this helps someone. I had this same thing, mine mostly seemed to die in the middle of the night, then lately started happening during the day. Then it finally happened while I was working. Then it became obvious what the real problem was - my ten year old UPS with a five year old battery. I heard it beep then everything shut down, but it should have easily held through the little power blip. I looked at its display and it showed only three bars of battery, but still plenty to hold for maybe 20 minutes. I then tested it by unplugging it, and immediately everything plugged into it died. I next noticed it seemed to be bulging a bit. I took it apart and found that the battery had greatly expanded. That was two weeks ago, when I bought a new UPS. Have not had a problem since.

I have been having the same issue with my iMac for past 2 years.

Replaced power supply unit....no change.

Replaced hard drive.....no change

Replaced main board ......no change

Replaced ram in diffrent slots..... no change

Replaced power cord and diffrent power sockets.....no change

Took into apple 5 times ..... no change

You name it I have done it. Bit frustrated by now and writing this I hope it will not shut down.

I think it might be a design issue. Will have a yet another conversation with apple support.

Please find a solution ........

M

I'm having the same problem. Do people really spend money over and over to have work done on these machines? I'm so unsure as to whether I should just presume it's unfixable and buy a new one. Let me know if you ever find any info that is helpful. Thanks so much.

Came across your reply to the topic when looking for possible fixes for my late 2015.

If you have an apple store close by to take it to, and you stay on taking it back in quickly, you will only end up paying for the part they replace that fixes the problem.

I had my computer in there 4 times over a couple months, and they replaced the logic board, back cover, and power supply. I only paid for the power supply. Still having issues though, and I dropped the ball on taking it back in so I'm guessing they would make me pay for the next repair since the repairs are only warrantied for 90 days.

A clean install should take software out of the picture. Have you run Apple Hardware Test

Apple Hardware Test:

https://support.apple.com/en-us/HT201257

It's not very reliable at finding errors but if it does report an error, it can be useful.

Yes, as I said AHT is not the greatest for diagnosing disk problems.

And you've run Disk Utility > First Aid on the disk?

Disconnected all peripherals?

Here's the Apple Support Topic on the subject.

OS X: When your computer spontaneously restarts or displays "Your computer restarted because of a problem." - Apple Supp…

If that doesn't help, take it any Apple Store for a free evaluation. They have far better ways to diagnose hardware problems.

EDIT: Oh, I forgot to mention that if the Mac hasn't been used in a while it could be the pram battery. You might try replacing it.

Wait until the Mac shuts down unexpectedly, then examine your system.log file to identify the numeric code associated with the shutdown. Read below.

Find the Console app - it is in your Utilities folder and looks like this

You can find it by selecting Utilities from the Finder's Go menu.

Open Console.

If the log list column on the left is not already displayed, show the log list by selecting Show Log List from Console's View menu.

Locate system.log in the list and select it. Many date and time-stamped entries will appear, hundreds of them, and you must find the entries relevant to your Mac's problem.

To do that type the words shutdown cause in the Filter field at the upper right of the Console window.

That will cause all log entries to be hidden, except for the entries containing those words. There will be a numeric code associated with the log entry starting with "Previous shutdown cause". What is that code?

Yes. Although Apple does not reveal each code's exact meaning, I have learned from observation that Previous shutdown cause: 0 will always appear if a desktop Mac's power is suddenly interrupted. I have never seen it associated with anything other than that. From those observations it is reasonable to infer the reason for the shutdowns is a power interruption, and not something that can be fixed with software.

The reason for the power interruption can be external to the Mac, such as a defective power cord, wall socket, or poor power quality in your home or electric utility. You can eliminate those potential causes by substituting a different power cord and / or a quality UPS.

Possible internal causes could be an intermittent connection or similar fault anywhere in its power supply circuit, which includes the path from the iMac power receptacle, to the power supply itself, to low voltage paths throughout its logic board and all the connections within. Those causes become much more difficult to troubleshoot.

If you are not successful using a different power cord and UPS then schedule an appointment to have Apple evaluate your iMac. The nature of intermittent problems is such that the likelihood of a prompt and accurate diagnosis is slim. Describe the problem in as much detail as you can, but be prepared for them to immediately conclude there is nothing wrong with it. If they do, insist they keep it for a while longer.

I hate to give you bad news, but your concerns bear a striking resemblance to this Discussion: Re: Re: Re: iMac shuts down randomly for no apparent reason. It involves an iMac model nearly identical to yours. Whether that problem was ultimately resolved or not is unknown, but please read my reply dated Nov 29, 2015 replying to mikestan56. It is on the last page, and might point to a possible solution for you. If you decide to read the entire Discussion beware that it contains lot of irrelevant and distracting crosstalk.

Please follow up with your determinations.