2017 iMac keeps suddenly shutting down

Progress report: last week in the Apple Store the third Genius I worked with managed to get the Fusion Drive, well, fused again - I was seeing the 2.12TB storage, for example. Yay! And they kept it in the shop for tests re: the sudden shutdowns. They called me a couple of days ago and said I could pick it up; I went in, and they explained they’d had the machine running at a high CPU rate for a while (I think?), and hadn’t had any shutdowns. I pointed out that the shutdowns had had nothing to do with high CPU load, and had often happened when I just had one app opened, and was simply in the middle of typing: the system shut down upon me inputting something, and wasn’t correlated with how much work the machine was doing.

I persuaded them to do more testing with it, and they should call me in a few days. Surely there must be some kind of record in the logs they can look at, even if I don’t know what to look for. I kept suggesting they check the power supply, even though I’m not 100% sure that’s even a thing, and I’m sure they’re sick to death of people like me who have read a few threads on the internet and therefore consider themselves experts.

Dunno if I’m being a jerk with it, but taking the computer back and forth is VERY annoying and I really don’t want to have to do it again, especially since there’s only one Apple Store in this city and it’s very busy and as far as I can tell it’s impossible to make an appointment. As in, every time I check, there are no appointments available over the next eight days, and they won’t let you look further ahead than that. At least this time they let me leave the empty box in the store, rather than trucking it back across the city yet again.

I picked up my iMac a couple of days ago; with the extended period of time testing, they still weren’t able to re-create the sudden shutdown problem. I’m not sure how they could re-create it - I have an image of one of those “water ducks” repeatedly tapping keys, because it was always some form of input on the keyboard, rather than CPU use, that triggered a crash. So the “fix” has been re-fusing my fusion drive - which may or may not be related to the shutdowns.

Anyway, for now the iMac is working well, and the Fusion Drive is presumably working properly (or at least it’s included when I look at the Storage tab), so that’s an improvement. I used the computer quite a bit yesterday and haven’t had a recurrence of the sudden shutdown issue, but that’s still not enough time to tell.

I’m still confused why there’s no way to look at the system logs and see “Oh hey the computer shut down because of a sudden loss of power these multiple times”, but this certainly isn’t the first time that computer diagnostics have confused me, and it won’t be the last.

To be honest my current fix is doing extensive writing on my iPad, rather than my iMac, because I don’t trust the iMac not to crash and wipe everything from the past ten minutes or whatever. Oh, and no more gaming, because why bother? Which isn’t a good place to be.

Anyway, I’ll report back in a few days on whether or not the shutdowns have return.

Follow up a month later: so far so good - since the Apple Store "re-fused" my Fusion Drive (or whatever they did), the sudden shutdown issue has not reappeared. Everything seems to be working as it should, and the computer by and large feels significantly faster than perhaps it ever has since I bought it a year ago.

But I did also shell out for Apple Care, which I've never done before. It's understandable that I'm a little worried about the stability of this machine going forward!

Etresoft, thanks again for the help on figuring out the nature of the problem.