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4 2015 iMac 27 i7 cuts out occasionally
I bought a late 2015 iMac 27" i7 on eBay. The seller didn't pack it properly so the screen arrived cracked from one side to the other. I kept it, got a refund, and replaced the screen. I upgraded the 256Gb Apple blade drive to a 1Tb m.2 using an adapter, and it's actually about 40-50% faster than than the Apple drive was. I also added a 2Tb SSD at the same time. It's been working great for over a year.
Recently it seems to cut out - totally stop running and the screen goes black - for about 10 to 20 seconds. Tapping on my touch pad seems to get it back for a bit, but then it cuts out again, sometimes after only 5 to 10 seconds. This is usually while I'm surfing the internet - YouTube... I get tired of it quickly and turn it off. Later, it boots normally and runs fine again. I run iStat menus, and the CPU and GPU temperature is never above 45 - 60 Centigrade, so not too hot.
Any ideas what is going on? Thanks in advance for your help and comments.
Update (10/12/25)
I understand your comments about Fusion Drive issues, except that I don't believe the computer was sold with a Fusion Drive originally, only the 256Gb blade drive. When I replaced the screen, there was only the blade drive in it, and there was no SATA cable for a second drive, although that might have been removed if in fact there had been a Fusion Drive there. It was running on only the original Apple blade drive when I first started it up with the broken screen, which makes me doubt the Fusion Drive situation. I've made my own fusion drives many times, and I think I understand how they can be "broken." This was not the case here.
I'm not sure about your comment that disk0 should be the boot drive. It's my understanding that that particular drive numbering is controlled by the OS, not something that is necessarily constant, (or user configurable for that matter) depending upon which drive is first recognized by the OS. That's why I was wondering why my blade drive wouldn't be seen first as it's directly mounted on the logic board rather than via a SATA port. In any case, I'm satisfied with the speed since it's actually significantly faster than the original Apple blade drive.
I set up TG Pro to increase fan speeds a little at higher than average temps, and it seems to be doing just that. I haven't run the computer enough to see if it still "blinks". Both iStat Menus and TG Pro give sufficient log data on temperatures to note that it's mostly the CPU, and to a slightly lesser degree, the GPU that are running HOT when the screen blinks out.
I have previously checked my screen with a flashlight when it's blinked and black, and there's nothing there. I think the whole screen is actually OFF. I've noted that the fan sometimes runs for a second or two, then it too shuts off.
crwdns2934109:0crwdne2934109:0
Let's see if the onboard diagnostics pops an error Use Apple Diagnostics to test your Mac if it does let us know (code & description)
Did you alter the screen settings? Like sleep or hot corners.
Which SSD is your boot drive the Blade or SATA drive? I do recommend you make a full backup of the boot drive as I think you'll need to reformat it.
crwdns2934271:0crwdnd2934271:0 DanJ crwdne2934271:0
I've done the Apple Diagnostics, and it came up with no problems. Zero. Huh.
Let me give some more information about my system. I am running Sequoia 15.3.2 using the OpenCore Patcher. As mentioned earlier, It's been running on the latest macOS for well over a year without this problem. I've also got an external drive with Monterey as the OS -- which is the latest natively compatible OS -- and it also gives the black outs on occasion. As noted earlier, this usually happens when I'm bingeing on YouTube video shorts. (Lots of interesting stuff there, and a LOT of Junk, too!) When I did my rebuild with the new screen and drives, I took the opportunity to re do the thermal paste on the CPU and GPU.
I went a little deeper into the temp history of my CPU & GPU (iStat Menu) and see that the temps on both have reached 100C on occasion. I'm beginning to think that I have to re do the thermal paste thing.
Any thoughts if that is the only thing I should be contemplating?
crwdns2934271:0crwdnd2934271:0 VikingShips crwdne2934271:0
I do have some hot corners, but the cursor is never near them when the screen dies. I do have one corner that puts the screen to sleep. As I've said, though, the computer has run perfectly for over a year since the rebuild.
My boot drive is the Blade drive, as this is where it originally booted from before my rebuild. It usually comes up as disk1 in a terminal 'diskutil list'. Does this mean anything? Somehow I would have thought that the blade drive would have come up as disk0 as it is "built in" to the logic board.
crwdns2934271:0crwdnd2934271:0 VikingShips crwdne2934271:0
@gmartinor - If this was a SSD Blade only system then the default drive ID is 0 for that drive. The order in which you setup defines the drive ID so if you left the Blade in then the SATA drive will be ID 1 if you took the SSD Blade out then that would have allowed the SATA drive to be ID 0 and then when you added the new SSD Blade it would become ID 0. The OS installer (Disk Utility) defines the Drive ID based on what it see's. So that explains why 'I suspected a Fusion Drive set here. As the SSD Blade is not seen as the first drive the SATA drive is.
With the flashlight test we are trying to deduce if the backlight power is the issue or the video data. OC could also throw a Panic error which looking at the log you might find. But given the displays wink-out I don't think that was your issue.
That leaves a power supply or logic board issue. If keeping the system cooler solves the issue then I would lean into a logic board issue and/or the M.2 adapter and the given M.2 SSD.
crwdns2934271:0crwdnd2934271:0 DanJ crwdne2934271:0