iMac shutting off and rebooting after a few seconds, looping

My son's iMac, model listed on the side, appears to no longer be functional since last night. This morning, we noticed that it kept rebooting over and over, only getting to the chime and the loading bar every 3-4 boots. Every other time it just shuts off and starts over after you can hear it spin up briefly.

I've tried resetting PRAM and NVRAM already, tried internet recovery, internet diagnostics, various other startup commands... to no avail. Even in Target mode it shuts off and starts the reboot loop over.

Before I take it apart or get used replacement parts to try and fix it (leaning towards PSU, pretty sure any potential software causes are ruled out)... I wanted to see if anyone on here has any tips or pointers as far as what the cause might be, which part might be causing it, or if it's even worth fixing for such a relatively old iMac. I was hoping it would last a bit longer, heh.


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@civrock unplug your hard drive and see what it does.


What's happening on the screen during all this?


@crowbot When it just shuts off after briefly spinning up and then rebooting again, the display is black. When it actually gets to the chime and load screen, it shuts off (and starts the boot loop again) at different points of the load bar.

An additional piece of info I noticed... when I completely unplug the iMac for an extended period of time, 15+ minutes, it appears to get to the chime reliably and even stays on a bit longer... I got it to the desktop once but it then just shut off again after a few seconds without any actions being taken; and the whole boot loop starts over again.


Since you get the chime and at least once got to the desktop I’d lean towards either a power supply issue or a motherboard issue. It may be a chip overheating and taking down the system. Opening it and poking around would be the next step.


I went ahead and opened it to see if anything obvious sticks out, however there was nothing other than a bunch of dust (which I cleaned out). I reseated the PSU cables just to make sure nothing was loose.

I put it back together but the behaviour remains the same. I went ahead and ordered a used power supply since it was relatively cheap and not too tricky to swap (compared to the logic board, more expensive and much higher level of difficulty to replace it).