Question Computer shuts down a moment after booting

[WhatsAComputerMachine]

This began happening spontaneously, (I think it was after a brief power outage but I’m not sure). After pressing the power button, motherboard LEDs light up and the fans start, but shut off after 1-3 seconds. I’ve replaced every part possible with some spares I have, including the motherboard, the CPU, the power supply, the RAM, and I tried booting the PC with only one HDD at a time and the same problem keeps on happening.

What could be causing this? How can I fix it?

 

[richiestang_78]

Does your mobo have LEDs to display error codes? If not buy a cheap mobo speaker(usually about 5 bucks) to hookup to it for a beep code. There's a header on mobos specifically for a speaker.

Swapping things blindly complicates diagnosing the problem since now you dont know if it was bad RAM, GPU, etc. It could also very well be a bad PSU.

 

[WhatsAComputerMachine]

The first thing I did was switch the power supply. That did nothing.
The motherboard does have LEDs and it showed that the problem was with the CPU. I switched the CPU and that also did nothing.
Maybe the problem was with the motherboard, so I switched that too.
Nothing.