iMac abruptly switches off at random

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Level 1

9 points

iMac abruptly switches off at random

For the past month or so, I’ve been having a problem with my late-2012, 27-inch iMac that I can’t quite put my finger on. In short, the computer switches off abruptly at seemingly random times — it doesn’t shut things down, logs out, turns the power off, etc. but just goes from working normally to off as if someone pulled the power cord out of it. Sometimes up to a week or so goes by without this happening, and at other times it does it several times in a day; it happens when the computer is being used but also when it’s asleep.

Turning it back on requires me to unplug the power cord, wait a bit, and then put it back in before the computer will respond to the power button.

Looking through the system logs doesn’t show anything useful. Here’s an extract:

15-09-15 19:41:50,892 com.apple.xpc.launchd[1]: (com.apple.photostream-agent[2562]) LaunchServices returned a bundle URL that does not match with the LoginItem's known association.

15-09-15 19:41:50,893 com.apple.xpc.launchd[1]: (com.apple.photostream-agent) Service only ran for 0 seconds. Pushing respawn out by 10 seconds.

15-09-15 19:44:06,000 bootlog[0]: BOOT_TIME 1442339046 0

15-09-15 19:44:55,000 syslogd[42]: Configuration Notice:

ASL Module "com.apple.AccountPolicyHelper" claims selected messages.

Those messages may not appear in standard system log files or in the ASL database.

15-09-15 19:44:55,000 syslogd[42]: Configuration Notice:

ASL Module "com.apple.authd" sharing output destination "/var/log/asl" with ASL Module "com.apple.asl".

Output parameters from ASL Module "com.apple.asl" override any specified in ASL Module "com.apple.authd".

When the computer turned off at 19:41:51 hours or so (that is, earlier tonight) I was using a web site in Safari, and as the log shows, I got it powered back up a bit over two minutes later. I’ve checked the logs almost every time after this has happened, and as far as I can tell there are never any messages about things going wrong that would cause the computer to switch off. I can’t trace the problem to something I’m doing or some program that’s running, because it hardly ever happens under the exact same circumstances.

I’ve pretty much ruled out that this happens because the computer doesn’t get any electricity: there’s a stereo and two external hard drives plugged into the same power strip, and these all keep running fine. I’ve also changed the power strip for a new one (and I mean a new one) without any effect. What’s more, I went into the system prefs and set it that the computer automatically turns on after power loss, but it doesn’t do that if it switches itself off. On the other hand, if I unplug the power cord, wait for about a minute and put it back in, it immediately boots up. If I plug it back in too soon though, the computer refuses to start, whether automatically or after I press the power button.

I ran the hardware test, did the extensive one, and it reported no errors. I’ve also reset the SMC — to be honest, I seem to have no choice but to reset the SMC if I want the machine to start at all after it turns itself off 🙂

All in all, I'm leaning toward the idea that there’s something wrong with the SMC, though. If it was the power supply then I’d think the computer wouldn’t need the power cord disconnected to work again, so it all gives me the impression that the SMC goes wrong somehow, kills the power to the whole machine, and needs a reset before it will work again.

So my questions, basically, are: am I right? If so, what can I do about it? And if not, what else could it be and how can I fix it?

iMac, OS X Yosemite (10.10.5)

Posted on Sep 15, 2015 11:28 AM

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Answer 1

Priztats

Level 1

4 points

Jan 24, 2017 8:16 PM in response to GurthIsMe

Hi,

I just wanted to check up on any news you had about your 27". I've been reading about this problem everywhere--and of course it's happening to me--and wanted to see the results you got by replacing your own power supply. That is, if you even still have Mac!

Thanks,

Hamilton

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Answer 2

rkaufmann87

Level 10

140,550 points

Sep 15, 2015 11:55 AM in response to GurthIsMe

By chance do you (or did you) install third party antivirus, cleaning or other maintenance utilities on the computer?

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Answer 3

GurthIsMe Author

Level 1

9 points

Sep 15, 2015 12:38 PM in response to rkaufmann87

Ah, the thing I forgot to mention 🙂 No, I didn’t. I’ve never in my decade of using Macs installed any of those, and most certainly not on this particular computer.

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Answer 4

John Galt

Level 10

141,150 points

Sep 15, 2015 2:59 PM in response to GurthIsMe

Though it is not possible to provide a definitive diagnosis absent a trip to the Genius Bar, the symptoms you describe are consistent with a failing power supply. The good news is that an iMac's power supply is one of the least expensive components to replace. Have Apple evaluate your iMac.

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Answer 5

GurthIsMe Author

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9 points

Sep 16, 2015 1:27 AM in response to John Galt

Is there a way in which I could check this myself, without having to take my computer away for someone else to look at it?

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Answer 6

John Galt

Level 10

141,150 points

Sep 16, 2015 8:00 AM in response to GurthIsMe

In the same manner as before, look for a system.log entry of "Previous shutdown cause:" logged subsequent to the "BOOT_TIME" entry following an abnormal shutdown. A numeric code will follow it. Negative codes indicate an abnormal shutdown associated with an abnormal cause, generally understood to be hardware-related. The two log entries in your original post do not indicate anything abnormal. As such they do not indicate anything you don't already know: your iMac simply shut down at random.

Short of replacing the power supply, you already described everything reasonable to identify it as the faulty component. If you are not doing so already, be sure to use a surge suppressor or UPS from a reputable manufacturer. That does not fix a faulty power supply, but it will obviate concerns regarding power quality. You ought to use one anyway, whenever expensive electronics are involved.

Assuming the problems continue, you can replace the power supply yourself. Apple does not sell the part directly but they are commonly available. You would of course invalidate any remaining AppleCare warranty. The power supply itself it not expensive, but be aware that disassembling your iMac is a royal pain.

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Answer 7

GurthIsMe Author

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9 points

Sep 16, 2015 8:03 AM in response to John Galt

Heh … “Sep 15 19:44:56 localhost kernel[0]: Previous shutdown cause: 0” — and the same for the other two in the current log.

I’d already looked into how to replace the power supply, and given that I’m old enough to remember hand-soldered computer add-ons (though too young to have made any myself), I don’t foresee any insurmountable problems taking apart this machine to do it myself. AppleCare I don’t have anyway, so no worries about that either.

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Answer 8

John Galt

Level 10

141,150 points

Sep 16, 2015 8:25 AM in response to GurthIsMe

GurthIsMe wrote:

“Sep 15 19:44:56 localhost kernel[0]: Previous shutdown cause: 0”

I believe that is what will be logged if you simply pull the iMac's power cord. Confirm that suspicion by doing just that. It won't be any different than what has already been occurring and won't cause any harm.

Although I did not mention it, be sure your power cord itself is intact. An intermittent internal break will not be obvious, so you would have to substitute another. The iMac uses the same commonly available IEC power cord as most other electronics. That would certainly be an easy fix.

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Answer 9

GurthIsMe Author

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9 points

Sep 19, 2015 1:32 AM in response to John Galt

I’ve swapped the power cord with that from another computer after the last switch-off, and am now waiting for when it’ll happen again so it confirms that’s not the problem 🙂 The one from this computer seems to be fine in any case, according to measuring it with a multimeter.

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Answer 10

yuffers

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Oct 3, 2015 10:16 PM in response to GurthIsMe

Wondering if El Capitan fixed this?

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Answer 11

GurthIsMe Author

Level 1

9 points

Oct 4, 2015 1:42 AM in response to yuffers

I haven’t installed 10.11 yet, but I doubt it would help given that this doesn’t seem to be a software issue. What I did do is replace the power supply, and I’m now waiting to see if this has really solved the problem or if I only bought an expensive look at the insides of this iMac.

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Answer 12

yuffers

Level 1

0 points

Oct 4, 2015 6:50 AM in response to GurthIsMe

The only reason I ask is because i also have experienced the same issue the past few months. Took it into a shop and they said it looked like my hard drive needed to be replaced (1tb-fusion).

I took it home for the weekend to back my stuff up before they did the swap (I didn't want to the pay 75 dollar charge they asked to back up my stuff). I also am hesitant on them doing it because it does seem it would be a power unit issue based off what others have mentioned (but who am I to go against what the mac shop says about the issue being the HD).

I installed El Cap a few days ago and so far my iMac (late 2012 model) hasn't shut down for the past few days with heavy use. It usually was shutting off after 2-3 hours of use. Sometime it would shut off after 30 minutes. I usually had to unplug the power cable for 15-20 sec then plug it back it to turn on.

Maybe El Cap helped but then again still not sure.

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Answer 13

GurthIsMe Author

Level 1

9 points

Oct 4, 2015 7:37 AM in response to yuffers

I’m not sure, but a failing hard drive seems a very unlikely cause to me for immediate shutdowns of the kind my computer was (I hope not “is”) experiencing. FWIW, the frequency with which mine shut off varied tremendously. On average it was once every couple of days, but there was one day where it did it multiple times (the worst was about ten times in a day — I lost count at around six or seven — one time even within five minutes of starting up again) while it also stayed on for the better part of a week before quitting again. Based on my own ideas and the replies above, I decided to swap the power supply and like I said, am now waiting to see whether it will stay up for the months I used to go between reboots before. Of course, I can’t tell what may be wrong with your computer, so if 10.11 does turn out to have fixed things for you, so much the better 🙂

BTW, if you decide to replace parts yourself, I found the iFixit guide to be very good, if maybe somewhat overly cautious — though better safe than sorry. You also don’t need to buy any iMac opening tools if you have an X-Acto knife or similar and some old PIN or credit cards handy, and you certainly don’t need the sets of adhesive strips that are sold for re-attaching the screen. I made the mistake of spending €20 on those, when all you need is a roll of double-sided tape, 5 mm wide and not too thick. Just remove the old adhesive (it comes off easily if you lift the edge with a knife), cut the tape to length and stick it down where the original strips were.

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