Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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iMac restarts when external monitor unplugged

This iMac will restart at the login screen or before it. First thing I tried was to reformat the hard drive and reinstall the OS with no luck. Basic SMC reset, etc.. have been completed as well.

Started with Apple Service Diagnostic (ASD) tests

Memory: Passed. Also took out each module individually and still restarted.

Hard Drive: Passed short test (long test was not completed). However, unlikely a hard drive issue. Computer will restart when booting off an external drive as well. Also pulled the HD while booting from external just to rule it out for sure.

CPU: passed tests

GPU: passed tests

Thermal sensors: Passed loop test (10 times).

Fan test: Fans passed tests (at all speeds)

*Does not boot in safe mode*

Opened the Mac up and cleaned a significant amount of dust out with air canister.

The iMac will stay on during testing without fans on full speed.

Checked the shutdown logs: kernel[0]: |Previous Shutdown Cause: -128

This is an unknown hardware error. Ram was checked and replaced with another set. Problem persisted.

UPDATE

The iMac will power on perfectly fine (without me having to make the fans spin full speed) as long as I have an external monitor attached. As soon as I unplug the monitor, the Mac will shutdown. This external monitor "fix" allowed me to check the temperatures with iStat and everything is perfect. It is definitely not thermal related. I am starting to suspect that maybe the integrated graphics have failed? Discrete graphics (Radeon) is being used while the external monitor is plugged in.

Answer this question I have this problem too

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What does the onboard diagnostic LED's show? I'm thinking you have a power supply issue.

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I'll check that out as well. I have thought about the power supply as well... but it would be strange for the power supply to be the problem because the computer runs perfect as long as the fans are set to full speed.

Oh, also it runs without the fans running full speed while using ASD (apple service diagnostics). I would think if the power supply was the issue, it would also shut down during these tests)

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onboard LED's are good by the way

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Another very strange thing to add. I just connected an external monitor (the original screen works perfect still). For some reason, connecting the external monitor (both iMac screen and external monitor are on)... the computer boots up and runs perfect. What is going on here? It is using the Radeon card without an issue. The power supply is clearly working. The thermal readings are all perfect.

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-128 userCanceledErr User canceled an operation

http://www.macwizard.com/errors.html

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Lets do a SMC & NVRAM reset does that make a difference?

Update (01/16/2018)

Your system has two graphics states low res (Intel Graphics 3000) & hi res (Radeon HD 6770M or Radeon HD 6970M). The switching logic between the two graphic services appear to be in trouble here. Basically your system uses the low res until an OS calls for the higher res. When you add in a second display Hi res is used either in mirror mode or extended desktop mode.

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All of that was done

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Yes, that is correct. So, it might be possible that the Intel graphics onboard has failed. By connecting an external display, the intel graphics are disabled during the boot process and therefore the computer boots normally. That might be my best guess at this point

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I can see that the Radeon card is being used under displays. So, it cant be the Radeon card causing the issues. I have also now been able to check iStats and all temperatures are good and all sensors are working perfect. When using full-fan mode, this data is unavailable obviously.

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Do you know of any possible way to disable the integrated graphics completely?

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Another update: system will completely shutdown as soon as I pull the external monitor out.

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I have exactly the same issue with my 27 inch iMac 2011. I connect a Apple LED Cinema Display and the moment I remove the usb and mini DV port the iMac reboots then goes into a rebooting cycle. As soon as you reconnect the led monitor the iMac reboots correctly and runs perfectly. It’s really strange.

Did you ever get to the bottom of your issue?

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I did a ton of work on my computer and crossed everything out except the logic board. Unfortunately, you are best using it with the external monitor instead of repairing. Another thing you might be able to do is hold down the power button down while plugging the power cable in. This will put the fans at full speed, but I was also able to use the computer without external monitors by doing it. External monitor is my preference.

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