How to determine cause of iMac "blacking out"?

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Hello,

I have a Late 2012, 27" iMac, running Yosemite 10.10.2

Lately my iMac has been shutting down on me (instantly) for no apparent reason. Doesn't matter what I'm doing, or what combination of programs I have running, it'll just go black.

Now, I'm pretty sure that when there is a kernel panic, you get the grey screen with Japanese writing stating that your computer has suffered a problem and that you need to restart it by holding down the power button, correct?
This is NOT what I'm talking about.
My computer will just literally go black for no reason. Sometimes it does it 4 times in 20 minutes, sometimes it doesn't do it for days.
This just started about 3 weeks ago, and nothing has changed in that timeframe (I can't remember installing any new programs or changing anything I'm doing).

One thing I do notice when it happens is that I can't just hit the power button and get it to start again. I have to unplug it from the wall for 10 seconds, then plug it back in. I know some of you are like, "Well, dummy, that's your problem." However, how come none of the other things plugged into that outlet shut down or reboot (items such as my scanner and external hard drive)?

Furthermore, I have my "Energy Saver" settings set to reboot after a power failure...which my computer is not doing whenever it decides to just randomly black out.

Lastly, I just ran the (extensive) Apple Hardware Test and it came up clean.

Any thoughts on how to troubleshoot this issue?

Thanks,

Justin
 
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chas_m

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Sounds like your fans have failed, and the computer is overheating. I would take that in to a tech and get that fixed before you destroy the machine permanently (not saying you're at fault here), but first -- make a backup!!
 

chscag

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2017 27" iMac, 10.5" iPad Pro, iPhone 8, iPhone 11, iPhone 12 Mini, Numerous iPods, Monterey
The Late 2012 iMac 27" has only one fan centrally located, however, I agree with member chas_m that you should have it looked at by an Apple genius. Make an appointment first.
 
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Sounds like your fans have failed, and the computer is overheating. I would take that in to a tech and get that fixed before you destroy the machine permanently (not saying you're at fault here), but first -- make a backup!!

Odd thing is, is that my fans almost never kick on (but they do, kick on), and I've never had the computer quit while they were running.
Also, when I ran the Apple Hardware Test and the computer was being put through its paces, THEN the fan came on (and yet the computer didn't crash for the entire 1h2m test).

If you still think it's the fans, I'll schedule an appointment.

Thanks,

Justin
 
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chas_m

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Well, let's back up a bit. You say first that the Mac is "shutting down" on you instantly -- a sure sign of either overheating, a bad temperature sensor, or a failing power supply. Later you describe it as "blacking out," which implies that the machine is still running but the display cuts out -- which would be a graphics card/chipset issue. Which is it?

Really, though, any of those possible issues warrants a trip to the Genius Bar.
 
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Actually I believe he did infer in a round about way that it goes black - not shutdown since he has to unplug it to restart it. It does sound graphics related and possibly memory needs reseating. If it goes black press the power button for 5 seconds and it will force it off. Have you Flashed the NVRAM?
 
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Well, let's back up a bit. You say first that the Mac is "shutting down" on you instantly -- a sure sign of either overheating, a bad temperature sensor, or a failing power supply. Later you describe it as "blacking out," which implies that the machine is still running but the display cuts out -- which would be a graphics card/chipset issue. Which is it?

Really, though, any of those possible issues warrants a trip to the Genius Bar.

Sorry, my apologies for the terminology in my original post.
I guess I shouldn't have used the term "blacking out" with something that has a monitor. I meant that term more in the human sense where you're going along and everything is fine and all of a sudden you just black out (ie: dead to the world).
So, just to be clear my computer absolutely shuts off entirely, out of the blue (it does NOT keep running and just shut the screen down...it shuts down entirely).

Sorry for the confusion.
 
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MacInWin

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As chas_m said, it's most likely power supply, or overheat or a bad temp sensor making it think it's overheated. A trip to the Genius Bar is in order. Make an appointment first.
 

dtravis7


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Really fast, my late 2007 was doing that. What solved it in MY case was a PMU reset. Never did it again after that.

PMU=Power Management Unit. There are pages that tell how to reset the PMU on your machine. Will try and find one.
 

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