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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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Random restarts/shutdowns - never when AHT is running!

Dear All,

I have been struggling with my attempts to fix my sister's iMac 27" mid-2011 (which used to be mine). The problem is very simple yet extremely difficult to pinpoint a reason: random restarts/shutdowns at any time between OS X boot and normal operation.

AHT used to show some error normally associated with the HDD fan or similar thing - however, yesterday I removed BOTH the original HDD and a third-party SSD and installed ONLY a new SSD from Samsung to see if the problem disappeared.

Alas, the random issues continue even though the SSD works fine AND the extended AHT shows no more hardware errors.

Having a look at some other threads, it may be that the culprit is the 6970M GPU (again); when booting in safe mode things seem to get better (but not always)...and even more interestingly, the machine NEVER suffers from the issue when running the AHT.

Now I am even thinking of putting the GPU in the oven in order to reball it. But again: why does it not happen under the AHT if it were a hardware issue? Besides, it doesn't seem to be temperature-related either, as it also occurs when I pushing the fans up with smcfancontrol.

Quite frankly, I am lost as to the cause (terminal normally shows the previous shutdown cause as -128) - is it OS-related? Is it the GPU? Is it the MOBO? Any help would be greatly appreciated!

Update (03/19/2017)

Tks a lot, Dan - in fact I have just installed a new SSD there with the OWC thermal sensor (in place of the original HDD), so no; I haven't actually faced an issue with the fans running at high speeds and so on. I was just trying to tweak them via smcfancontrol in order to see if there was a problem with the fans themselves (there isn't).

And just after another random restart I ran the extended AHT again - and again, no hardware issues reported. What I am still trying to find is whether this is system/software-related or not. Once more: when I run the AHT for several hours, no random restarts or shutdowns occur. It is ONLY when either OS X is running or perhaps the GPU is used more heavily - I have no idea.

Update (03/20/2017)

Crash file

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Have you reviewed the crash logs? Is there anything their?

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The stated reason for such shutdowns is -128 - not really helpful, I guess...

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Can you post the crashlog here so we can see it.

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Here you go - these are the extracts I could get before another random restart:

WARN: XPC SERVER (com.apple.SetupAssistant/10.10): No keychain access at end of session setup, will attempt shutdown...

Mar 19 00:49:54 tests-iMac.local locationd[54]: locationd was started after an unclean shutdown

Mar 19 01:08:21 localhost locationd[54]: locationd was started after an unclean shutdown

Mar 19 01:08:22 localhost kernel[0]: Previous shutdown cause: -128

Mar 19 01:36:18 tests-iMac.local sharingd[792]: 01:36:18.662 : SIGTERM received, shutting down.

Mar 19 01:36:19 tests-iMac.local VDCAssistant[165]: shutdown_server_connections: Detaching application from window server

Mar 19 01:36:19 tests-iMac.local UserEventAgent[254]: shutdown_server_connections: Detaching application from window server

Mar 19 01:36:19 tests-iMac.local com.apple.AmbientDisplayAgent[172]: shutdown_server_connections: Detaching application from window server

Mar 19 01:48:08 tests-iMac.local reboot[1859]: SHUTDOWN_TIME: 1489884488 876887

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To start with you can't run this system without the OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade. It maybe too late here if you've been running the system with fan control software as it tends to burnout the fans and stresses the logic board as it can't manage the heat the way the system was designed to do. The in-line sensor is needed when you replace the original Apple drive with either other HD or SSD.

I would install TG Pro (full version here) so you can monitor the temp just when the system craps out. Unlike other monitoring apps this one has a log file so you can catch the temp just at the point of the shutdown.

Update (03/21/2017)

I've appended your Q with your log output

OK, lets give this a try go into Safe Mode and see if the system is able to come up clean holding down the Shift key.

If it does startup we'll need to create an external bootable drive as we'll need to redo the format. Hopefully you have a backup of anything important, if not do it now.

Here is the guide you'll need to follow to create an OS installer: How to create a bootable macOS Sierra installer drive. A 16 GB USB thumb drive will work. I've not tried setting it up in Safe Mode so you might need to use a different system to set it up.

Once done boot up under it using the Option key to select the USB drive. Here we want to use Disk Utility to completely remove the partitions. Then run the installer it will create the new partitions (hidden & main).

Once the installer has completed the OS install we want to exit it (don't migrate anything over from your backup). Apply all of the OS updates and then download any App Store apps you might have bought. That way you have fresh copies. Once done run the system is it stable?

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Tks a lot, Dan - in fact I have just installed a new SSD there with the OWC thermal sensor (in place of the original HDD), so no; I haven't actually faced an issue with the fans running at high speeds and so on. I was just trying to tweak them via smcfancontrol in order to see if there was a problem with the fans themselves (there isn't).

And just after another random restart I ran the extended AHT again - and again, no hardware issues reported. What I am still trying to find is whether this is system/software-related or not. Once more: when I run the AHT for several hours, no random restarts or shutdowns occur. It is ONLY when either OS X is running or perhaps the GPU is used more heavily - I have no idea.

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Thanks a lot, Dan - actually I had already done this before reading your advice. Interestingly, the iMac lasted a bit longer whenever booted in Safe Mode - and before that I was able to clean install El Capitan with a USB stick even though the random restarts were still taking place.

I also took the opportunity to reset the PRAM a couple of times and install TG Pro.

Now the weird thing is: suddenly the iMac stopped suffering from such random restarts even in normal boot mode and with many apps open at the same time, including Youtube on Safari, Grapher, GPU Test and so on (in order to stress the GPU as well). Uptime right now is about 2 hours, something that I could not even begin to consider before.

Yet the exact reason is unknown - I am/was absolutely sure that it had something to do with the GPU and/or GFX drivers, or perhaps the PSU under stress - now I am not so sure. Any ideas?

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