Macbook Pro A2289 EMC 3456, MLB 820-01987-A, shorted U6903, stuck at 5v

Dear friends,
a close friend of mine gave me this machine, she told me that the last days the fan was going high and that it was heating up too much for the usual things, and that suddenly one morning it didn't start anymore .
I opened it and inspected carefully with a high magnifying lens, found no oxydized spots but a short on PPBUS_G3H , then a blown U6903.
I changed U6903 with a new one, PPBUS_G3H became 558 (measuring with red probe to gnd , and black on F7000) , PP3V3_G3H_RTC 463.
Now when I plug the usbc cable (original Apple 61W power supply), nothing starts automatically, if I press the power button nothing happens, and these are the measures I have:

(Impedance with no powersupply - voltage)
PPDCIN_G3H 702 - 5.06
D3370 PP20V_USBC_XB_VBUS 130 - 0.00
PPBUS_G3H 558 - 12.00
PP3V3_S5 351 - 0.26
PP3V3_G3H_PMU_VDDMAIN 398 - 0.02
PP5V_G3H_LDO 0.00
PP5V_G3S 421 - 0.01
C3200 PP3V3_G3H_RTC 313 - 3.21
C3205 PP1V5_UPC_XB_LDO_CORE 487 - 1.54
C3208 PP3V3_UPC_XB_LDO 508 - 3.21
C3105 PP1V5_UPC_XA_LDO_CORE - 1.54
C3108 PP3V3_UPC_XA_LDO - 3.21
C3102 PP1V8_SLPS2R 389 - 0.36
R3023 UPC_XA_VDDIO_CFG - 1.55

I noticed that PP1V8_SLPS2R is 0.36 but I don't know if it should rise before or after CD3217 switches PPDCIN_G3H to 20v.
It seems that the mainboard is stuck at 5v, I tried to revive the T2 firmware but other macbooks don't find it connected or in DFU mode (using the rightmost port looking it from left side, the farthest from the screen) .
I tried to connect the DFU port to a Macbook Air updated to MacOS 14, but Finder doesn't show any macbook connected, when I tried a simple Apple power supply chargin cable, OS asked to connect "the correct Thundebolt cable".
I tried a complete thunderbolt cable, a simple Apple power supply chargin cable, and a data+chargin cable, but the mac isn't seen...
I tried to force the DFU mode following instructions on Apple website but nothing helped.

Please, what can you suggest to measure or test?
Many thanks

Correction:
PCB should be marked 820-01987-A, not 820-1987-A.

Regarding your measurements:
You mentioned that PP1V8_SLPS2R = 0.36 V — actually, the T2 should bring this line up before switching the CD3217’s PPDCIN_G3H to 20 V.

If you want to verify the power sequence correctly, start with three basic checks:

Current draw – measure with a proper USB-C tester (it will show if T2 tries to start).

Resistance to ground – begin from the power rails feeding the T2 power domain.

DFU state – forget Apple’s official tools for now and try the Force DFU method. There are guides online that explain how to trigger it manually; it’s worth testing if your T2 can still enter DFU mode.
Edit :
I’m a bit pessimistic here — I’ve seen several boards stuck in the same stage.
It looks like the T2 doesn’t perform RAM initialization.
If you have a microscope, check the DDR power rails (PP0V82_SLPDDR, PP1V1_SLPDDR) and the signal lines around the T2 package — sometimes you can see short pulses on those caps when T2 tries to start RAM.
If these rails are missing or dead-flat, T2 will never reach the DFU or power-on sequence.

Hello Ado,
thank you for the correction, the board is actually 820-01987-A, I can't edit the name of the topic, but I hope that an admin can do it for me so it can be used for future searches.

THank you also for the suggestions!
Sadly at this time I don't have an usbc meter, I'm in the process of buying it but I can't choose one that has a right balance between cost and reliability...do you have a suggestion about?

Reguarding the board, are there other T2 power rails in addition to those that I already have taken?
PP3V3_G3H_PMU_VDDMAIN 398
PP3V3_G3H_RTC 313
PP5V_G3H_LDO 485 (this was missing the value in the first message)

I've tried to force DFU through the SE032 pads but since PP1V8_SLPS2R is 0.36 I think it can't work this way...unless I force 1.8v with an external source on that rail

Since the main short originally was on the PP3V3_G3H_RTC rail, could this have damaged the U7800 or U3100/U3200 logics?

Hello Ado,
after many days I've bought a usbc meter and I will attach the photos when it's connected to the two usbc ports, so you can read its values (that are constant in time and doesn't have pulses to 20v or other voltages).
As you can see, current absorption difference is around 7 mA, but D+ and D- are quite different, does this suggest anything?





At the same time, thank you for the edit, I've made other measurements around the RAM voltages (meter in diode mode, red probe to gnd and black one on pin, at side its voltage) :

PP0V82_SLPDDR 278 0.00
PP1V1_SLPDDR 327 0.00
PP1V25_SLPS2R_SMC_AVREF 613 0.27
SOC_COLD_RESET 464 0.36
PP1V8_AWAKE 362 0.00
PP0V9_SLPDDR_SOC_PCIEREFBUF 85 0.00
PP1V2_AWAKE_SOC_PCIEREFBUF_F 538 0.00

I took these measures taking the mainboard out of the case and probing the back side of the T2/U7800, withou battery and all other flat cables, should I unsolder the metallic cover of the T2 and take measurements with all the cable and battery connected?
Anyway, in this status they don't have pulses, they are flat...does this mean that the T2 is brain-dead or can be some other thing?

Hi Vogelek23,
thank you for the tip about the topic name change and for the suggestion about my case.
I'm having difficulty in finding one U7800 for this mainboard, do you have an idea where I could buy one from?
At the moment I don't have donor boards from which I can take it out