820-02100 low p3v8_aon

Lady's a2485 just stopped.
No visible liquid damage to board
5volts 20mA not connected to battery, trackpad
ppbus_aon 12.3v
pp3v8_aon oscillating 2.5v to 3.4 avg 3.2v (low)
I noticed U5700 has an erratic input voltage pp5v_s2_main 0.6v to 5v
p5vs2tps_pwr_en 0.2v but Im going around in circles thinking this could be caused by low pp3v8_aon

Ive replaced U5700, no change.

Im lost

U5700
8 - PPVIN_P3V8AON - 12.3V
7 - PP5V_AON_P3V8AON - 5V
5 - P3V8AON_PVVC - 5V
12 - P3V8AON_PWR_EN_R - 4.5V

Check diode mode to ground on all secondary power rails.
Pay atention to SSD power.
Compare readings with good board, or reference table.

I do happen to have another non-working board - measurements from this in brackets at end of line

pp5v_s2. 0.3v close to OBdata 0.39v
pp1v2_s2 0.2v close to OBdata 0.26v
pp1v8_s2 0.22v close to OBdata 0.255v
pp1v8_aon 0.4v close to OBdata 0.44
pp3v3_s2 0.37v close to OBdata 0.4v
pp2v5_nand0 0.265v similar to OBdata 0.302v
p1v2_lx0_nand0 0.295v close to OBdata 0.305v (0.256)
p0v9_lx0_nand0 0.124v significant less than OBdata 0.345v (0.167v)
p0v9_lx1_nand0 0.124v significant less than OBdata 0.345v (0.167v)
ppvin_p3v8nand0 0.3v significantly more than OBdata 0.161v (0.189)

Are any of the last 4 rows enough to suggest anything?

Sudden death is many times caused by failed NANDs; even no short detected.
If CPU can't access its dedicated partition on SSD, it can fully boot.

Does it need access to the NAND before getting to 20v? This board does not get to 20v.

You can test disabling a SSD power rail from working board.

I tried that. After turning off pp2v5_nand0 the other [20v capable] non-working board still got to 20v.
So I suspect this would indicate that reading a NAND partition is not required before going to 20v.

Can any other area explain pp3v8_aon oscillating 2.5v to 3.4 avg 3.2v?

I had one A2141 switching to 20V with shorted SSD (2V5_NAND), but I was not sure about M1 machines.

Possibly problem with a secondary power supply.
You should check all of them in diode mode; including master/slave rails.