iMac 5k 2014 won't boot/turn on Looking for ideas on how to fix my iMac, it's a 5k 2014 that will no longer boot. It all started one morning when I'd put it to sleep the evening before, when trying to wake it, nothing happened, and the power button did nothing too. 

Randomly managed to get it to boot later that day, would reboot fine, but when I turned it off it no longer would switch on again. 

Took it to the apple "geniuses", were able to replicate the error in the store, and said they'd run some test, but suspected it would be the power supply unit. Had a call the next day, with them saying they could no longer replicate the issue, and all of the status lights inside were fine. So I picked it up, when I got home, no power, and same issue… tried a different kettle cable and different sockets, no joy… later that evening I again managed to get it to boot (pressed power button, then it booted about 10 seconds later, as I was walking away from the machine). I upgraded to High Sierra, just to make sure it wasn't software related, restarted a few times, all seemed fine, but again when I shut it down, it refused to boot up.

Took it back to Apple, again were able to replicate the issue in store (wouldn't boot), but once again they called the next day saying they couldn't replicate the issue, but would replace the PSU. I've just been to pick it up, and guess what… exactly the same issue, no power when pressing the power button! The shops now closed as it's Sunday, so will have to wait to let them know i'm still having issues, but wanted to check if anyone had any bright ideas on what else I could try? I've tried:


Taking power lead out for 15 seconds, putting back in, waiting for 5
seconds, then pressing power (doesn't work)
Removing all RAM from the system, won't boot
Tried 3 different sockets in the house 
Tried 2 different power leads


Any other ideas?
Ended up having the PSU replaced, but the issue still presented itself (wouldn't boot). Fixed in the end by replacing the motherboard.