PC shuts down and don't want to turn on. Again.

First off, I built this computer for someone and i have no idea what they were doing with it (the owner is a typical younger cousin). I have warranty for all parts. Specs: Gigabyte H310M S2P 2.0 Corsair Vengeance LPX Black 2x4GB 2400MHz CL 14 GTX 1650 Zotac Gaming D6 OC i3-9100F Toshiba P300 1TB SPC Vero L3 500W

The story so far:

The diagnosis last time, I mean, a month ago, indicated a dead motherboard. It wasn't my diagnosis. I gave the motherboard for warranty, the new one came from the service, or at least my examinations indicated it. I mounted it, checked temperatures (0,5h of OCCT), hdd (SMART) and give back to the owner. GPU was sitting in my pc, so I know it was fine.

It was 2 weeks ago. Yesterday it died again. The same story, it shuts down and don't want to turn on. I have no idea what the hell is wrong with it and I'm pissed a little bit. Today I go to get that pice of junk, but I want to test it on place and maybe... maybe fix it without bringing it to my home (It's kinda far).

So I plan to: \- check voltages on PSU \- change ram/psu to my own \- examine motherboard for leaks \- change hdd to my m2.

I have AMD cpu so, I can't test cpu/motherboard. Any more ideas what to check? Oh, and extension cord was kinda sus (they connect 2 cables to make it longer), but I don't think it could cause the problem.