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Then check L3700 and R3741.

Then your problem is on U5200 area. Check R/C/Q5230/40 and their traces. Be sure all resistors around U5200 are good.

Try using Command + F1 (+ Fn).

Sorry, 8.5V...

Missing 5V_S2SW_USBC01? Compare voltages and diode mode readings between UF/G800 areas.

Remove F5200 again and check C5254 voltage.

U7100 requires a good communication with SMC, to control PPBUS_G3H properly. If both SMC and U7100 have correct reset signal and good continuity on SCL/SDA lines, the voltage should go 12.5V aprox.

Pin 2 should be 1V aprox; check R3742 and traces.

Did you test again starting in Target Disk mode, after DFU revive was completed? Check if its SSD appears on other Mac.

https://drive.google.com/file/d/11GqPXvcb9xNv6MB-lQBNy6WuzWnDvMW9/view?usp=sharing Check on Internet for more files; maybe on badcaps forum.

Test external video disconnecting internal LCD. You should get the main screen. 3V3_S2 is present?

https://drive.google.com/file/d/11sLmbfCr-9AexCUL7Hne_bF0qOMSF5q-/view?usp=sharing I don't remeber if is good for sure...

Post ALL U3740 voltages.

???

Why do you thing missing? Do you know what these signals represent? What is actual PPBUS_G3H voltage, w/ and w/o battery connected?

Just in case, try known good BIOS. Don't forget to make backup.

Replace U2890 too, from same board; it helds the firmware required by CD3217 chips. Anyway, make a backup of the working one (from donor); usually we reflash the chip from the board under repair. A cheap CH341 programmer is good enough; just need a board adapter to solder the chip on it. BTW...

When I say battery bus, I mean SCL/SDA lines; many-many times asking about such readings. For U7100 is more easy to write pins number, instead of signal names. Try another ISL6259 chip. If still the same replace SMC.

Reflashing BIOS, just in case.

Check page 46...