2015 iMac 5K doesn't boot ANY partition, recovery or Internet Recovery

[stooovie]

Any attempt to boot ANY kind of macOS fails and the computer reboots (with the startup chime). I've tried the installed 10.13.3 from the internal Fusion drive, clean MacOS install from external SSD, Time Machine and Internet Recovery. Everything fails the same way. What's interesting is that there are no Recovery partitions on either the Fusion or the external drive available in the boot menu, even though I'm 100 % positive they were there.

Resetting PRAM or SMC doesn't help, nor does disconnecting anything and removing/swapping memory modules.

Either way, it looks my iMac hardware is pretty hosed, right?

EDIT: I DO ocassionally manage to boot into the primary HS installation, but it reboots within seconds or minutes anyway. Looks like some kind of overheating maybe?

 

[Juicy Box]

I had a similar issue with my 27" Late 2012 iMac with the 1TB Fusion Drive.

I had problems that looked drive related, but both the HDD and SSD kept passing the Apple Store's HW diagnostic test. They would reformate the drive, only to have the same issues pop back up a few months later. Months turn to weeks, but my complaints fell on deaf ears. I knew it was something wrong with the Fusion Drive, but they wouldn't replace it since the HW test always passed.

12 days before the end of my AC warranty, the HDD portion finally fully failed. Nothing would boot, internal, external, recovery, internet recovery, nothing....

I took my iMac into the Apple Store convinced that they would replace the Fusion Drive, and I was right they finally did after the HW test showed it as failed.

But, they couldn't get anything to load either, for 2 hours... Finally their HW diagnostics test booted after multiple attempts, but nothing for two hours of trying.

I know this doesn't give you an answer, but it could be similar to my problem.

Maybe if you could tell us the issues leading up to your Mac not booting, if there were any.....

 

[stooovie]

I yanked out the power cord for half an hour and now it doesn't even power up :-o So it's probably the power supply or something.