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R1931 communicates with R1800. However on the other side it goes directly to the CPU and I do not have any TP to check... It look likes it have a TP but it does NOT.
PCH requires high level (1V) at CPU_VCCST_PWRGD and CPU_VR_READY, apart of 3V on pin 3/U1950.
If you get pulse on these, PLT_RESET_L should pulse too.
If not, check all voltages from page 11 (pulse at least); be sure all series resistors and coils are good there.
If no LIO board, or trackpad connected, fan goes to max in 10-15s.
You should test the board alone on the desk for USB activity.
Also good to have a spare Air screen for test; I use broken one, to check backlight presence.
You didn't mention anything about advanced corrosion when started the thread.
SMC should be reballed in such case.
Passing board trough ultrasonic bath may help; especially in case of accumulated crap under CPU, RAM, or other BGAs.
Be aware, incorrect UC may damage more.
I already reballed the SMC and didn't work
I do not have ultrasonic cleaner for this board =( What did was cleaning some pads which I tought had a bit of corrosion. It's not that bad but its what I think is making the board sometimes have PLT_RESET and even sometimes making it have fan spin working and after following as you requests.
What do you mean by UC?
After all our chat what would you consider and look with more care? RAM doesnt look bad to be honest