27 iMac (2012) crashes and requires smc reset to turn on

I'm guessing here, but I think there is some sort of very transient temp spike that evades the sampling window of those programs but tells the defective power supply to shut itself off. This could be a total red herring, but it is all I got.

What would cause a heat spike when the computer is asleep and it is not connected to anything so there are no processes running?

Why has replacing the power supply solved the problem for so many people?

Why has the problem started for so many people between the two and three year mark after purchase?

From what little I understand, replacing the power supply with a LiteOn power supply has solved the problem, but not for the others. (No one answered my question above, so I am still not certain about this, but it is the impression I got in the first 36 pages of this thread.)

I think the most likely explanation is that the power supply Apple uses is inherently defective. As for your other questions, it is possible later versions of OS X unmasked the issue, or maybe it is a capacitor that starts to die or something like that. I don't know enough to offer anything other than guesses.

When you spend the amount of money I did on a computer you expect it to be usable for longer than two and a half years.

I could not accept that the way to solve the problem was to spend lots of money on randomly replacing components in the hope that would fix the problem. Others have done that unsuccessfully and spent more money than what their computers are worth.

Apple don't seem to care that their continuing denial about this abrupt shutdown problem is a PR disaster.

We are all in the same boat here. That is the reason I don't want to bring mine to Apple for repair. But I have had two well-regarded Apple-certified third-party repair places say they don't want to touch this can of worms.

The fan control at least seems to have applied a bandaid.

After reading the posts here and elsewhere I came to the conclusion that this recurrent shutdown issue is caused by a defective power supply.

I ordered a LiteOn power supply from the UK and had it shipped to Australia.

I had also broken the main body hinge so got one of these as well.

My OWC temperature sensor/hard drive cable was faulty too, so I got a replacement for that as well.

Yesterday I stripped down the iMac to an empty chassis, replaced the broken hinge, power supply and HDD cable.

I spent over 4 hours on the job, proceeding slowly and carefully using the guide from iFixit.

If you proceed carefully it is not all that difficult. You just need to get the adhesive strips from iFixit and some Loktite if you are doing the hinge.

Stripping off the adhesive with turps from the LCD and iMac body probably takes the most time. You also need to be very carefull removing and replacing the various cables.

So far everything is working perfectly. (Touches wood!).

Another update from me. It should have been close to a year that I got my power suppy replaced by Apple (so not a Lite-on one apparently) and it has not shut down once on me requiring a SMC reset. It is definitely power supply for me. It may be hard to persuade Apple guys to replace the power suppy and they insist on keeping the old one or give you that faulty one back for a fee. Funny I know.

I sport a 27'' i7 iMac with 680MX

@oomoot :

Thanks for that update. That is really good to know. I think I will just have to bite the bullet and take it to the moron bar and get it fixed. This is just driving me absolutely nuts. I have several iMacs that are older, both at home and at work, including two from 2007, all of which have been flawless. They really blew it with this one.

I mean the fix is relatively cheap for an official repair from Apple so I'd do it. But even better is getting it fixed elsewhere with a Lite-On PSU. I'm in Turkey so I just didn't want to wait for a PSU. I also was not sure if it was PSU so I thought getting it to the Apple Store for diagnostics first is a good idea (in which they of course failed to find anything wrong like almost everyone else in here). I almost had to beg over the phone for them to swap it for a new PSU (make no mistake, I paid for it, but still...)

The upside is I just had to haul the iMac once to the Apple Store. It really is a pain to carry it.

Another thing is I use my iMac seldomly. Some other people reported that the shutdowns are coming back after around six months with the original PSU replacement. As I've inquired newer iMacs were also using the same PSU so maybe they fixed something with them maybe more quality capacitors I don't know. My replacement is probably the one that they also use for the retina iMacs.

Shortly, get your PSU changed and you'll most like be fine.

So this started happening to me about 3 weeks ago on my iMac late 2013. Computer randomly shutting off no matter what I am doing (internet, video games, anything), and at completely random intervals.

Took it to Apple and they told me I needed to replace the power supply, so I reluctantly did that. Got it back a week later, and all was fine for about a week of it being on, and then 3 days ago it shut off again. Then again the next day. Then twice today.

I got a refund for the power supply they replaced, but they said the next step was the logic board. I am most definitely not paying for that since I see here that it didn't work for a lot of people. I really don't want the computer to stop working again in a few months after spending $700 to fix it.

Seriously peeved since I've had this $2100 computer for 2 years and it's already broken. I didn't get Apple Care + which I am seriously regretting now.

Hi there - hope I'm on the right thread for this reply.

I recently suffered the same issue for over a week whereby my late 2012, 27" iMac (1TB fusion drive) would unexpectedly shutdown like someone had yanked the power supply cable from the back. I could only get it restarted by removing the power lead for 15 secs, plugging back in and waiting 5 secs before starting up - and apart from a Chrome "unexpected shutdown" message, you wouldn't have known that it had shut off.

I spent a few hours on to Apple Support trying various solutions (Diagnostic test, MemTest, SMC/PRAM resets, Sierra reinstall etc).

By trial and error, I eventually identified the problem's source as being my Satechi 7 port USB 3.0 hub which had 2 x Porsche design (1TG and 4 TB) external HDD's attached, along with 2 x charge/sync docks for iPad and iPhone. Since I removed the USB Hub and plugged the peripherals into the back of the iMac, I have had no issues whatsoever - and my pride & joy is back working like a dream.

I hope this update helps

It's the identical fault and thanks for your solution.. Hopefully that was it. It can be a cause but my feeling is the non Liteon power supplies are not 100%. I bought a new Liteon from eBay got it replaced by a tech and that is the only thing that solved it. The issue is however intermittent for many. In your case, if it does return, which I hope not, I suggest finding a Liteon supply.

Add another victim to the list for this problem. Late 2012 iMac 27". A month or so ago my wife start complaining about her iMac just shutting off for no reason at all, then not being able to power it back on. This unit has been plugged into a APC UPS since it was originally purchased. As with numerous other posters in this thread the only way to reset the iMac is to pull power and let sit for a few seconds. Sometimes it requires pulling the power cord a second time. Very surprised to find the number of people that have had this problem here. To think how many hundreds more just take theirs in to get it fixed without reporting it is somewhat disturbing.

Since I don't have an Apple store nearby, I have decided to just order a replacement power supply from an Amazon seller. Description says it is a Lite-On model so fingers crossed. Just wanted to make sure to report here to add to the documentation.

Thanks for the report. Mine is still driving me nuts. I did a whole disk wipe and reformat, and it was stable for 4 days, so I falsely assumed it was ok. As soon as I started trying to populate the disk with files from backup, the problem reappeared. This makes me worry it is a combination of the power supply and fusion drive. I might replace both.

I had my late 2012 iMac in several times over many, many months with no solution because they could not replicate the issue even after keeping the computer sometimes running for 2-3 days. The problem got worse and worse. Finally, it did fail there and the Genius Bar concluded it was the entire mother board. After some very unhappy back and forth over quality and what I and they should be responsible for regardless of age, they agreed to cover half the cost. I had it replaced about a year ago and it has been running flawlessly ever since. It was either that or buy a brand new unit, which I did not need. This is definitely a common defect in this run of 2012 iMacs...