So I'm tasked as the family and friends PC technician because I build and maintain my own shit. The computer I'm trying to repair would not boot for more than a few seconds before powering off. I figured it was the power supply since it booted the same way without anything connected, ram, gpu, etc. I got my RMA'd PSU in and it boots for a minute now but powers down afterwards. It doesn't send any signal to the monitor. Tried booting without the gpu, ram, etc. I replaced the cmos battery as well, still the same shit happens.

To me this says its the CPU but I dont know how to be 100% sure. Any troubleshooting tips?