2015 27'' iMac loop reboots, then doesn't power up

[sciuro]

Short and sad story.
My iMac suddenly got into a reboot loop yesterday with either not loading completely or loading all the way into login screen but then rebooting again. I tried NVRAM reset which did not help; then I thought SMC restart would help so I unplugged the power cord for a good minute. Well, since I plugged it back, my iMac just does not power up at all. No sounds, screen does not light up, nothing.

What are my options? Could this be a motherboard failure, power supply died, RAM battery failure... anything else? With all Apple stores/Genius bars closed, I'm not sure how to approach this How much money am I looking into losing with repairs?

 

[DeltaMac]

Unplug the power cord. Unplug all other cables, too, just in case there is some power fed in from another device that has its own power. (unplug it all) Leave for an hour or two (overnight is fine.)
One more thing to try: Remove ALL the RAM sticks.
Plug in power cord (only), and try the power button to see if there is any response of any kind.
You should at least get error tones (because of no memory detected). If you get that, unlug power, reinstall memory (maybe one stick at a time, then try again)
If you get nothing at all, then you can expect a failed power supply (replace the EFI button battery while you are there, just as a test) - If a good power supply doesn't fix, well, it gets expensive - probably the logic board then.

 

[sciuro]

Done that already (unplug overnight, test without RAM - just as you described it). Nothing.
Not looking forward to getting under the hood of this machine...

 

[imac-rumors]

I had a somewhat similar experience. Random reboots, until one day iMac would not start. After doing all the tricks, took it to an Apple store. They replaced the PSU but it didn’t fix it. They said it was probably a logic board issue. Replacing it would be around $500 plus labor - still cheaper than finding a used board and doing it myself. The iMac has been sitting in a box since then. And I am still wondering whatever happened to it. Overheating? I never did anything power intensive on that machine. Design flaw? No idea... But very disappointing nonetheless. 27" iMac 5K 3.5GHz Quad-core Intel Core i5.

 

[Juicy Box]

Maybe a boot drive related issue? Do you have a Fusion or HDD?

Also, have you tried doing the Apple Hardware Diagnostic Test?

Use Apple Diagnostics to test your Mac - Apple Support

Apple Diagnostics, formerly known as Apple Hardware Test, can check your Mac for hardware issues.

support.apple.com

 

[imac-rumors]

In my case, it’s completely dead - would not power up at all.