xbox one x fast beep on / off

[Themi]

got given an xbox one x as dead, no pressure to fix as has been replaced and owner doesn't want it back

symtoms: press power button, white light, beep on and off almost immediately

checks:

• dead short on MEMIO test pad (doubt this is good but I don't have another board to compare against) injected voltage 1v nothing showing on thermal
• checked under thermal during power on only slight heat in SB, everything else front and back of board cold
• no shorts on any other test pads MEMPHY etc
• has standby 1.1v, 1.8v, 3.3v, 5v
• psu good has 12v
• no 12v rail short
• no short on 1.1v, 1.8v, 3.3v, 5v
• test shorted EN to 3.3 VIN on LDO voltage regulators and get 1.1v and 1.8v
• no short at retimer or encoder
• no short at ethernet
• ports in good condition
• no damage / burns / holes to board components

board was tested bare no attached components (drives, etc) only power button board
tested from bench psu

I am guessing APU, SB or SB nand ?

any ideas? I don't know what else I can do to proceed further