Gigabyte GA-Z87X-UD3H On and Off Behaviour

I have a Gigabyte motherboard where I plug in the 24 pin connector only it stays turned on.
If I plug in the 8 pin CPU power connector, it turns on for 3 seconds and turns off again.
What is the cause of this issue?
I have updated to the latest BIOS already

Short on power circuit 12v

EDIT: VRM or wotever… only getting ur hands on it and trace it if attempting repair.…f/download.html

Faulty VRM powerstage?