Xbox one no NBcore voltage

Hello. I got an xbox one fat here that does not boot. When pressing power button the led turns on the immediately off, same for the power supply, orange, white, orange. I have done a ton of theese, and I have been able to find the order in which the buck systems start. Here is what I know happens after pressing the power button:
First - Ram, U4F1 (top of board) and U6C1 (bottom left) turn on. (I get good flat voltages)
Second - U5E211 and U5F3 turn on (one of which is NBcore) and the U4F3. (no voltages or enable voltage)
Third - U9C2 (cpu and gpu core) turns on. (never gets to this step)

This xbox starts everything in the first step, and voltages seem good. They stay on for 300ms, then turn off. There is no enable voltage at all on the second step (al, three chips share an enable line). Not even a spike, checking with an oscilloscope. So some SB “check” must fail during the first stage, before the SB decides to send the enable voltage to the second step.
Does anybody know what checks are done before the second step of enables?
I have already manually enabled these three systems, inserting 3v on the enable pin line, and two output the correct voltages, U5F3 does send a couple pulses to the mosfets, but not enough to get a “normal” voltage, I am guessing since it boots for a fraction of a milisecnd. I have already tried replacing this last ic and got the same outcome.
Sorry for my poor technical terms.