For the past six months I've been trying to isolate sporadic iMac system crashes unsuccessfully. The machine is a late 2012 iMac 3.4 GHz i7 with the 3tb fusion drive (replaced awhile back under the recall program), 32gb of Mac certified RAM from Crucial, and the NVIDIA GTX 680MX graphic card option. I sometimes hook it up to an external monitor. I also typically have the machine hooked up to external USB drives. The system can run for weeks without issues, then crash or sometimes mere minutes/hours.

The behavior is that the system simply shuts down with out warning.

Things I've tried:

  1. Running the machine with a UPS to eliminate the potential for power fluctuations.
  2. Running the machine without a UPS to eliminate the potential that the UPS is to blame.
  3. Checked the RAM using a multi-pass scan.
  4. Replaced the RAM with new RAM directly from Crucial despite no issues found in scan.
  5. Scanned the hard drive and found no errors. (System uses 3tb Fusion Drive)
  6. Reinstalled OS w/ restore from Time Machine backup.
  7. Upgraded the OS.
  8. Ran the machine w/o the 2nd monitor hooked up.
  9. Ran the machine w/o the external USB drives hooked up.
  10. Left machine with the Genius Bar twice and allowed them to run multi-day scans on the machine with no issues detected.

The only thing I haven't tried is to reinstall the OS from scratch and just manually restore everything rather than restoring from a TimeMachine backup.

Today it crashed again. I'm at a loss and close to just junking the machine.

I know I should be able to determine something from the Console logs, but I'm not sure what I should be looking for and the volume of information in these is overwhelming.

How would you go about determining exactly what's causing the crash? Is there a string I should be searching for in the logs that would give me this information or any software I can run to monitor the system health to capture this at crash-time?

  • Welcome to Ask Different! :) Just curious, but have you found a way to bring on a crash at will? – Monomeeth May 17 '18 at 3:10
  • @Monomeeth Unfortunately, no. – Dave May 17 '18 at 16:35
  • You never described the crash...is it a hang, do you get a kernel panic, is it a reboot. The term "crash" is very broad. – Allan May 18 '18 at 0:32
  • @Allan Immediate power-down. It's as-if somebody pulled the power cord out of the wall and the system is immediately off. Upon reboot, there's no error screens, or indication that anything wrong happened. – Dave May 22 '18 at 16:53
  • 1
    That's not a crash, that's a loss of power. See this Q/A to diagnose and possibly solve. apple.stackexchange.com/q/212457/119271 – Allan May 22 '18 at 17:03

How would you go about determining exactly what's causing the crash?

Since you haven't found a way to bring on a crash at will, and the whole problem appears totally intermittent (i.e. it runs okay for weeks sometimes), it's somewhat harder to diagnose.

Normally you could go through a process of elimination to troubleshoot this, but that's not going to really work for if you haven't found a reliable way bring on a crash.

What you've done so far has been spot on, and doing a full fresh installation of macOS may be warranted, but before doing that there are some other things you could try. The most obvious one is to boot your iMac into Safe Mode as follows:

  1. Fully shut down your iMac
  2. Restart your iMac
  3. Immediately press the Shift key and keep it down
  4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).

At this stage I would normally recommend testing to see if the problem still exists. In your case that's not going to be easy, so I would just use your iMac in Safe Mode for a while until you have a reason to reboot (e.g. not only features are available in Safe Mode, so you may need to reboot at some point anyway).

NOTE: Don't be concerned if it takes a while to boot into Safe Mode, as your Mac will delete a whole heap of caches, verify your boot volume, etc, and this takes time.

Is there a string I should be searching for in the logs that would give me this information or any software I can run to monitor the system health to capture this at crash-time?

The answer to this can differ depending on the version of macOS you're running (since macOS Sierra, the Console app is an entirely new beast).

For now, as a starting point, I suggest you download EtreCheck and run it (it's free and safe to use). It will produce a report of your system. Look over that and see if it finds anything of concern. If so, update your question to include anything that it flags as a potential problem and, while you're at it, also include your version of macOS and how much free space you have on your startup drive.


The resolution to my problem was to replace the internal power supply. Apple performed the repair for $67 in parts and $79 in labor. The iMac has been rock solid since the repair was completed.


I had this problem and Have found a suggestion from a friend to have cured it completely. when I log out of my iMac I unplug it from the mains. I'm told this allows a capacitor which normally sets some important things in the running of the iMac. This should be reset when you login, however, this capacitor is not ding allowed to lose it's charge when the logout.

I have been doing this for a few days now and it hasn't suddenly out of nowhere changed to a dark screen and a need to login again. I hope this works for other people as well.

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