Question Motherboard or PSU???

Dec 31, 2021
Ive narrowed it down but if Im wrong please let me know.

I was given a gaming PC from my brother that had been damaged during a cooling leak. It was minor but enough to stain the graphics card. Stopped working completely.

I took it apart, cleaned everything with an ultrasonic cleaner, put it back together. No life. Used a power supply tester and it immediately came on. Assuming motherboard issues now. PSU shows perfect numbers but L.L for +12v.

This is where Im stumped.

Multimeter shows everything is 11.9, 5.9 and 3.9. Tester shows the same except +12v. I put the 4 pin power supply in the motherboard and ran a wire from the 15-16 pin. Life again! Cooling fan came on, liquid flowing again. Sounds strong. CPU LED shines bright red through the green LED strip lighting of the computer tower. CPU, both RAM sticks have red lights -__-

So here the question. Is the PSU bad and not producing enough power (+12v L.L and 11.9v on different tests)..
.. OR ..
Is the motherboard fried because it doesnt do anything plugged directly in? But the 4pin connector powers the fan and cooler while plugged into the board, only when the 15/16 pins are together or tester.

Maybe the pins on the main power supply are dirty from the leak? What would keep the pc from powering up now?

Its a X470 gaming plus mother board.