Random restarts/shutdowns - never when AHT is running!
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!
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.