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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:

enter image description here

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?

  • The verbose message indicates that what's happening is called a Kernel Panic. It's the Mac equivalent of what would be a Blue Screen of Death in Windows. It may be a hardware issue. – Wowfunhappy Nov 18 '16 at 4:19
  • Have you tried booting from another Mac started up in Target Disk Mode? – Monomeeth Nov 18 '16 at 6:47
  • @Monomeeth I didn't try that, but wouldn't it be the same as booting from a known good USB disk, which I did try? — The failing iMac will itself start in Target disk mode, but via its Thunderbolt ports, and I didn't have cables or another Mac with a Thunderbolt port handy to connect it to. Even so, that would only let me look at its internal disk, that doesn't seem to be the problem because the same error occurs when trying to boot the external USB disk. — Fortunately, it's still under Apple Care, so I'm going to try and bring it to an Apple Store today. – ElmerCat Nov 20 '16 at 19:52
  • Not necessarily as troubleshooting is a process of elimination, so starting up from another Mac in Target Disk Mode helps eliminate possibilities (i.e. Helps narrow down the problem). However, if you're still under Apple Care I'd definitely go down that route - let them narrow it down for you! Hope it all goes well. – Monomeeth Nov 20 '16 at 20:06
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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.

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