Surface Pro 7 - SLP_S4 present, then gone

[Prott]

Hello fellow Badcappers,
I have a problem with stubborn Surface Pro 7. I narowed it down to disappearing SLP_S4# (gone after around 200 ms).

Everything else works as it should (the whole power on sequence succesfully finishes, all the voltages are present) until this signal goes missing - with it missing the AND gate causes ALL_SYS_PWRGD to go back low and this makes VCCIN drop to 0 V as well. Interestingly the SLP_S4_DRV# signal from SAM is still present for a second, even after SLP_S4 is no longer there. I wonder what could either cause PCH to stop sending the signal or what could deassert the signal after such a short time (before even attempting to read the bios).

It doesn't seem to be caused by a bad bios because there's not enough time to even show any activity on the CLK signal for the flash.

Also, SLP_S3# is still present for additional few seconds even after S4 is LOW.

Does anybody have any tips on what to check / probe? Thank you so much.

[Prott]

UPDATE: So the whole power up sequence is ok up until PLTRST# remains LOW. PCH receives SYS_PWROK, but there's no change in PLTRST#. Is the PCH / CPU toast or is there something else that might be going on?

Resistance of PLTRST# to GND is 65 ohms (which seems to be ok when compared with other Surface).