iMac random restarts - still stumped? The definitive diagnosis?
The issue of random restarts on my (now old) iMac mid-2011 continues, as does the struggle for a proper diagnosis of the problem. This used to be my main desktop some years ago; later I sold it to a family member, who after a couple of years started reporting this problem. Then I took it back so I could try to fix it for good (I love a challenge). Yet the issue remains.
So I am writing to this knowledgeable community so i can try to, once and for all, find the answers I need. Now let me explain again what the problem is and what has been done already (I had already posted the original question containing “AHT” elsewhere here):
PROBLEM: Random restarts in the iMac mid-2011 (3.4GHz i7), 16GB RAM (Crucial) - they happen virtually anytime between/during boot time and normal operation, and most often right after the login is entered or a little after the Finder is active. Please note that this is NOT a random SHUTDOWN issue; but just random RESTARTS.
NOTABLE EXCEPTIONS: it NEVER happens when you invoke the AHT test (D boot key) or when acting as target in target disk mode (for instance, through FireWire); it also does not happen when booting in ctrl-R (recovery mode), except on a few occasions in the middle of a macOS reinstall on the main drive.
ACTIONS TAKEN (OR EXCLUDING FACTORS) - Virtually everything has been tried so far:
- Changing households (to exclude power outlet issues) - it has happened both in my house and my relatives';
- changing power cord - since this was originally bought in the US (and I currently live in Europe), I've tried swapping power cords, also to no avail;
- erasing/reinstalling macOS - several times with High Sierra; apparently this seems to stabilize the functioning of the iMac for a while, but then the problem comes back again;
- replacing the PSU - even though the first one I ordered had an issue with the temperature sensor, a good replacement part came and the problem still continues;
- changing the boot drive/using a different SATA connection - thinking that perhaps the SATA cable may cause the problem, I reinstalled macOS on another SSD with the second SATA connection available in the iMac - it seemed to resolve the issue for a few minutes/hours, but now it is back again;
- removing/reseating the added RAM - the problem continues;
- Apple Hardware Test - with the original and then bad replacement PSU, an issue was reported with the temperature sensor; afterwards, with a good PSU, no trouble is indicated; this EVEN after running the overnight extended tests;
- GPU - I can confirm that no graphical artifacts/banding have ever occurred with this iMac; not to mention that the AHT shows no trouble at all;
- fans/sensors - all seems to work fine now, so this does not seem to be the cause of the problem;
WHAT ELSE, THEN?
Given the above, the only remaining factors that I could remotely consider are:
- Major logic board issue - improbable, since nothing has ever been reported on AHT;
- bad primary SATA connection still plugged to the first SSD - even if it is no longer the boot drive, it might be that the bad connection causes random restarts just because it is still active somehow;
- extreme sensitivity of the PSU to power spikes/oscillations, thus requiring use of a UPC unit - highly improbable since the problem has been reported in different households; yet not totally impossible;
- software issue somewhere? Funnily this is what I tend to believe in right now, since the problem does NOT happen at all when doing the AHT or acting as target disk mode. But who would be the culprit? I have not tried installing Windows/Linux on it, so this is unclear.
IN CONCLUSION: I hope this helps others facing the same issue, and assists us in collectively debating the root of the problem. Thanks!
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