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Full ASD 3S155 EFI + OS should be around 2.7GiB, it should work on all 820-3115. Download from the usual Russian torrent. (boards with Mojave BootROM update may not boot ASD EFI but ASD OS will still work) You can also use MRI 64-bit.

No you must use same RAM chips as what this board model was shipped with originally. RAM chips must match SPD profile stored in EFI. A given board can come with a few different chips model, correct profile is selected with RAMCFG strapping resistors. Anyway not sure why you're trying to replace...

Voltage on PP3V42_G3H, SMC_RESET_L and PP3V3_S5_SMC_VDDA?

If PCH is warming up with only +3VALW present, and +3VALW is 13 ohms to ground, I'm pretty sure the PCH itself is partially shorted somehow, and it would also explain why PM_SLP_SUS# doesn't come up. In my opinion next step would be replacing the PCH (if you have a BGA rework station).

13 ohms to ground on +3VALW is too low. I'd guess it's caused by the PCH.

Is +3VALW present before pressing power button? +1VALW is never present right? What's the resistance to ground on +3VALW?

EC may be waiting for PM_SLP_SUS# to come up. Power sequence isn't clear on that. And of course it's handled by EC firmware so hard to know what it's doing.

Power button must be triggered for Core 2 machines with NVidia MCP to turn on. Shorted Q7030 or bad soldering of U7000.

Several reports of RIO board being the problem (apart from the usual OS and EFI corruption). Also one comment on Apple's forum (at https://discussions.apple.com/thread/252033190 ) that has since be deleted mentioned RIO board too and especially HDMI IC. There's also an SPI ROM for SD card...

I'm not sure of the exact power sequence requirements but I think at this point PM_SLP_SUS# should be coming up. What's the resistance to ground on this line?

820-01958 is the MBA 2020 (yes Apple always has to choose confusing numbers), schematics/boardview not available yet, 820-01521 can help indeed.

Bad keyboard most likely.

Check voltage on PCH_RTCRST# and PCH_SRTCRST#. Use component placement diagram to find and measure voltage on RH515, RH83 and RH84. https://www.badcaps.net/forum/showpost.php?p=987679&postcount=5

Voltage on PCH_DPWROK, PCH_BATLOW# and PM_SLP_SUS#?

Read the sSpec from the PCH package. On the picture at the Aliexpress link it's in the corner on the left next to "003" and reads SR30W. If they match it's good. (also I'd be a bit more confident buying from see-ic but for Intel chipsets in general it's not too much of a problem buying from less...

Fan won't spin until CPU gets burning hot on this board.

Post a picture of the board where you found this board number.

UB300/UB400 work in master/slave mode. The master CD3215 (UB300) must be able to read the firmware from the TBT ROM (UB090) for its 1.1V BMC LDO to come up and then USB-PD negotiation to start. The slave CD3215 (UB400) gets the firmware from the master CD3215 (UB300). So if UB300 doesn't work...

Liquid damage on LCD cable. Inspect inside connectors for damage.

Check the voltage on the LDOs as well.