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When running on battery, can you get access to USB devices?

You have a problem on that port with missing LDO_BMC. Compare diode mode readings around U3100 with another port. S-UEM2 can help you with some checkings there. https://www.masterliuonline.com/products/s-uem2-mac-usb-c-and-edp-meter

https://openboarddata.org/?a=showboardsolutions&bpath=laptops/apple/820-02773 BTW, use external switch method to enter DFU mode. https://logi.wiki/index.php/DFU_Mode_Restore_(Macs)#DFU_Mode_using_external_switch

Check MGL/ND/AMR_1V8 signals with lid open/closed.

Inspect USB-C connectors for corrosion. S-UEM2 can help you with fast diagnose of USB-C lines. https://www.masterliuonline.com/products/s-uem2-mac-usb-c-and-edp-meter

Any history available?

00165 should get 0.120-0.150 in diode mode there. Be sure Q7130 is the correct model. Post exact C7098 and C7140 voltages.

Just for test, replace LT400 with a jumper. Be sure all LT20x from page 85 are good. Check if you get correct power rails on JT201.

You should check DCIN/BMON_ISENSE instead. Their voltage depends by the current consumption from the charger and battery charging. 2V looks too low for PBUS_VSNS_OUT; it should be close to 12.5V. You should get 1.33V aprox at PBUS_VSENSE.

What is exact 820-xxxxx code of your board? What damaged component found on the mainboard?

Use ticker cables to inject voltage. You must heat the board, not the cables. Also use powerfull lab PSU and set amp limit to max first. Then start to inject with 1V; increase, if needed. If PSU gets in limitation mode, may not inject anything. If it supports CC mode, that helps.

"no caps lock LED working" You should follow KBD_CAP_CATHODE trace; driven by UT105. Seems that machine restarts only when more CPU cores are engaged. So you may have a problem with CPU itself. I recommend to inspect all I/V sensor circuitries, from pages 59-64.

You should do any other check possible before reflowing the chips. If you suspect GPU chip, you can lift (one side) all LA710-40 coils. In case if Intel side is OK, board should stay on, getting even USB activity.

Still looping if LCD is disconnected? Test the board alone on the desk. Check diode mode to ground on all big coils. Compare readings with good board.

Inject why??? You don't have short there, nor even low resistance. Just in case, replace F7140.

First of all, welcome to the forum! What is the exact ohm value of the short? How are you sure that actually inject 10W there?

If you can complete DFU revive, there may be a chance to save data, if needed.

Just need to fix U9500 problem and replace the NANDs.

Any history available? Tested w/ and w/o battery connected?

Try to access DFU mode, using external switch method. https://logi.wiki/index.php/DFU_Mode_Restore_(Macs)#DFU_Mode_using_external_switch