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Pin 8 is 3.3V output for AD/DA converters from SMC.
That is a reference voltage and must be very stable.
If that voltage is not present, usually board can only start in SMC bypass mode.
U5110 could be bad itself, or even SMC.
But you must start checking the filter caps.
Recently had problems with one Air 820-00165 board.
As it only started in SMC bypass, I waste lot of time checking all sensors output level; all found to be good.
Finally, I remebered that I forgot to check AVREF_SMC presence; such a simply thing.
Once I changed C5125 (found to be shorted), board started properly...