HP 8000 elite 4 beeps problem (component level trubleshooting)

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I have a Hp compaq 8000 elite ssf system that was given to me.
I do not know the history of the computer but i really want to get it running.
When the system poweres on, the front fan spins up to 100% and stops, folloved by 4 beeps and a flashing red light. (The power supply fan never spins up)
After a while of investigating, i concluded that it is eather the motherboard or the power supply.
I tested the power supply by manually switching it on and loading it with a few power resistors (7A load on a 12V rail) It performed fine exept power good signal, from the expirience i know that it is usually 5V but on this one I get only 2V, alltought it might just be a poprietery design. I tried feeding 5V and even disconnecting the power good wire, no difference. I tried disconnecting remote vire, got no fan spin and the same 4 beeps.
When it comes to the motherboard, I tried different RAM, I didn't have a compatible CPU but when I start it with no CPU i get 3 beeps and no attempt of startup.
Monitoring CPU VRM while the computer is starting, I get nothing. None of the VRM mosfets are shorted.
I don't know what to try next.
There is a video in witch you can see the behaviour of the main 12v and 5v sata rails and the power good signal when the computer starts.