Question PC won't turn on, how to be sure what the problem is before asking for replacement?

Jul 5, 2019
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PC is 2 months old, bought all parts and built it myself. It was working fine until yesterday when it refused to turn on.

Nothing happens when I press the power button: no leds, no fans, no beeps.

I tried to turn it on by shorting the power button pins on the motherboard, but couldn't.

Removed cmos battery to reset cmos, nothing.

Checked cmos battery with multimeter, normal 3v voltage.

Checked psu 24pin cable with multimeter, all voltages are correct.

I think the motherboard is at fault, how can I be sure of it?