How to isolate cause of iMac System Crash 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:


Running the machine with a UPS to eliminate the potential for power fluctuations.
Running the machine without a UPS to eliminate the potential that the UPS is to blame.
Checked the RAM using a multi-pass scan.
Replaced the RAM with new RAM directly from Crucial despite no issues found in scan.
Scanned the hard drive and found no errors. (System uses 3tb Fusion Drive)
Reinstalled OS w/ restore from Time Machine backup.
Upgraded the OS.
Ran the machine w/o the 2nd monitor hooked up.
Ran the machine w/o the external USB drives hooked up.
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?
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.