iMac shuts down randomly for no apparent reason

Same issue - random power offs that were becoming more and more frequent - and it seems I have finally SOLVED IT! Found the information I needed on another forum. This individual suggested getting a UPS to put between the iMac and the outlet. The other option - and the one I'm using - is to simply shut the computer down and *unplug it* every night when I'm through for the day. Have had ZERO mysterious shut-down issues since doing so. Apparently a slightly hinky outlet can leave a small static charge in the iMac's PSU & that leads to the power-offs. Hopes this helps others with similar issues.

I'm experiencing random shutdowns of my iMac. Brought my Mac to an Apple Reseller and they got it two weeks and could not find any problem. TechTool Pro can't find anything. Today he shut down 4 times. Even when I'm not working with it.

iMac 27" Late 2012

3,4 Ghz Intel Core i7

8GB ram (original)

GeForce GTX 675MX 1024 MB

When the iMac has shut down, I have to unplug de power cord for about 5 minutes, otherwise he won't start up.

If you have any external hard drives plugged in, unplug them for a while and test. That was found to be the problem with my late 2012 early this year; the OWC hard drives chips (which control the USB and/or FW ports) were not compatible with the newer firmware update in the Macs and caused it to shut down. After much back and forth between Apple Engineering and OWC, OWC replaced my hard drives under warranty with models which had different chips. Haven't had a problem since.

That I do not know - I only connect mine when I need them and then of course, they're powered on. But it's worth a try to pull the plug and test - won't hurt anything. FWIW, when this was happening here, I contacted Apple - they sent some diagnostic reports to their Engineers and they told me it wasn't the Mac, it was caused by the external. So when I contacted OWC, they actually told me (after some prodding) that Apple had changed something with a recent update causing problems with some older chips in their enclosures. So, if you test and have no problems without the hard drive connected, contact LaCie and ask specifically if their chips in the enclosures are compatible with the latest Mac updates.

This is what OWC said:

This is a known issue with both the Oxford 944 and JMicron 355 chipset in 10.9 or later.

This was back in Feb/March of this year, so in the meantime, those might have been updated in newer hard drives; older drives may still have the older chipsets (or they may have been updated - don't know if that is possible; OWC said they'd replace the chipset or the entire drive which is what I opted for since I was covered by their warranty).

This may or may not be relevant with anyone's problem here, but if there are unexplainable shutoffs and Apple says it's not their problem, it's good to look at all other possibilities.

We've been plagued by this for the last 8 months. My wife's 18 month old imac was shutting down randomly. Only an unplug/replug/power on got it running. random activities happening when shutdown occurred. 4 phone calls to apple support and 2 visits to the genius bar. Finally, last visit to the genius bar, Apple decided to replace the power supply, even though their testing dis not seem to indicate that was the problem. The support guy on the phone said, "It sure sounds like a hardware/power supply issue to me, so let's fix that". Got the computer back with a new power supply almost 2 weeks ago. No shutdowns since. It sure seems to be solved!

Rather than confronting him, better might be taking it all directly to his supervisor. Then learning if a better informed tech is available.

You can verify it yourself by examining system.log, but be sure to examine the archived logs also: system.log.0.gz etc.

I've made a screenshot - will look at it again tomorrow as I've shut the iMac down for the night.

Do you have an "anti-virus" product known as "AVG" installed?