iMac Won't Boot Any Disk — Keeps Restarting I'm trying to fix an iMac that won't boot any disk. 

The Verbose Boot option is on and a few lines are logged, but then it stops. A couple of seconds later, the computer restarts all over again with the bootup chime.

The restart happens so quickly I couldn't read it all, but captured a photo of the display:

  

I've reset the SMC and the PRAM. Holding down Shift for Safe mode, or Cmd-R for Recovery mode does the same thing.

If I hold down the Option key, I can get to the Startup Manager screen showing the "Macintosh HD" and also a bootable USB (El Capitan Installer) disk. Whichever disk I choose to boot from, the same thing happens. 

If I hold down D for the hardware Diagnostic mode, it runs for about 2 minutes and reports "No issues found". From there, it tries to boot into Recovery mode but the same failure occurs.

Have I tried everything?
It's definitely a hardware problem — either the system board or the video card.

I took the iMac to an Apple Store today. It actually passed all their in-store hardware tests, but the technician still couldn't get it to boot and agreed it was a hardware issue. He'd never seen a problem exactly like this before, and said there wasn't even a standard Apple repair code for it.

Fortunately, it's still covered by Apple Care and they'll fix it for free — they said it should take about five days to get parts and perform the repair.