MacPro early 2009 isn't getting power.

Hello, My MacPro is the early 2009, A1289. The dual CPU one.

It's been a long time I try making him work again. I already try to make the minimum config, no processor tray and cage with everything that have to come out, shorting the sys power on the board, but nothing. The 5v STBY is always on when diag led is press, the PROCA and B led power on for 0.5 sec when pug and unplug. Today I tried looking onto the PSU voltage with a multimeter and the pinout of the PSU. When I short the pin 4 and 14 I hear a click and the 12V is here, so the PSU isn't totally wreck. But yea, when pug back in nothing make it spin to life. I tried with the other MacPro. I have (the same) to switch the processor board, but no spin to life after that. And when I put the processor board of the faulty one in the working one it spin to life normally, it isn't the graphics card cause work on the good one.

The backboard is receiving power, the psu is good when the right pin is short, the processor board isn't faulty, could be the backboard half-break? But there are no signs of damage. Never left in dust, well take care of. No sign of water or electrical damage.

the reason i'm trying to make it work again is that i'm passionate about computers and i'm going to make a living out of it. for my homelab this mac is a good start for what i want to do right now.