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After days of troubleshooting I have finally found out exactly what is wrong with my Mac Mini: SLEEP and/or simply turning off the monitors causes a kernel panic that sends the Mac Mini into a boot loop. This can happen even when I'm booted to Recovery, Internet Recovery or even an OS X USB installer. As such, I think it must be a hardware issue, but I'm not sure which hardware. My questions are:

  • What piece of hardware controls sleep and/or monitor sync in a mid 2011 Mac Mini?
  • How can I troubleshoot this to find an exact cause of the error? I have tried booting to Verbose mode (Apple-V) when it boot loops, but it doesn't stay on the kernel panic screen.
  • Is there a way to get the Mac to stop on the error when booting? Something similar to Windows booting to F8 > "Disable automatic restart on system failure"

When I run the terminal command syslog | grep -i "previous shutdown cause" it shows the cause as "-128" but that is a vague error: overheating (not that) or "undefined". Thanks.

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    I've read through all of your troubleshooting re the other questions you posted. Can I ask, have you tested this behaviour with both another display and cable? This would eliminate two other possibilities (i.e. display and cable) as somehow causing the problem. Also, as far as I'm aware a -128 error actually represents a userCanceledErr event. In other words, the system thinks the user has cancelled an operation. What's your source for this being a vague error: overheating (not that) or "undefined"?
    – Monomeeth
    Nov 14, 2016 at 23:53
  • Thank you, @Monomeeth. I will see if I can find the link that referenced shutdown cause -128 being undefined. Also, I have not tried another display and cable. I will do so! Nov 15, 2016 at 2:31
  • I can only find this, but I thought it was an official Apple site: georgegarside.com/blog/macos/shutdown-causes Nov 15, 2016 at 2:36
  • Noteworthy post (I've tried all suggestions here except replacing the RAM): apple.stackexchange.com/questions/212809/shutdown-cause-128 Nov 15, 2016 at 2:39
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    Thanks for the references. I've never seen the -128 error referred to in that way, nor have I seen that site before. I suppose one could argue that a userCanceledErr event is a bit vague as users would know if they had in fact cancelled an operation (intentionally or not), so it's not clear what may have triggered the system to interpret it as a user cancellation. Interesting! Still doesn't explain why this would result in a kernel panic though. Let us know the result of testing another cable and display.
    – Monomeeth
    Nov 15, 2016 at 5:49

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