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Bad solders, broken pads/traces. The mainboard may be affected by the heat applied on the replacement process.

https://logi.wiki/index.php/DFU_Mode_Restore_(Macs)#DFU_Mode_using_external_switch

"SSD_RESET_CONN_L, SSD_SR_EN_L?" Checked these voltage levels?

"smc_rst_in- 3,45v" Bad! Check U5274 voltages; on its pads, if not soldered.

When I ask about DIODE MODE, I expect something like "0.xxx". Don't write end "V" in such case. Multimeter reads the voltage induced by itself on the mainboard; in diode mode! Is not a voltage reading, like when power is applied. Avoid any confussion, please. Do you mean OL, or 0.000 for XA_CC1/2?

"Injecting 5V directly into the USBC Power Input line (PPVBUS_USBC2)" Why do you think that can help? Do you know how USBC Power Delivery Protocol works? The charger never injects 5V directly there, in the first moment! CC1/2 pins are the first involved in charger detection. Did you check diode...

Resistance is not low enough to heat anything; unless a thermal avalanche occurs.

Any history available? SSD_RESET_CONN_L, SSD_SR_EN_L? Compare readings on U3740 with good board.

"since injecting voltage directly into PPVBUS_USBC2 had the expected behavior" I don't understand what this means. You must replace SN25A chip together with its TBT ROM from donor board.

We don't have a clear description of all DFU error codes. I saw threads on the forum where people got DFU completed after days of traying. How can an error code help in such case? You may have problems with CPU itself...

So you don't get PPBUS_G3H voltage with charger alone. That is very important information and MUST be mentioned in the first moment. Post U7000 basic voltages (charger alone): P_IN, AUX_DET, SMC_RST_IN, VDDA/P, EN_VR1, SMC_RST*, A/BMON.

Check CC1/2 lines (diode mode) on the bad port.

Looks to have a big difference between pins 1 & 4. Compare readings with good board.

"w/ and w/o battery connected; charger present ..... w/ and w/o charger" ???

Cuando apretas boton de power, notas la caida de SMC_ONOFF_L desde 3V a 0V?

I expected you to check SCL/SDA lines TO GROUND, in diode mode; not over Q6950. In other words, check diode mode to ground on Q6950 pins 3/6 and 1/4.

Let's clarify the situation. Machine works with charger, but doesn't charge battery? Or the machine never turns on? Check PPBUS_G3H and 3V3_G3H voltages w/ and w/o battery connected; charger present.

"on usb voltage tester nothing, black screen" Didn't consider that important to mention in the first post? Check diode mode to ground on CC1/2 lines.

You must check reaction on pins 1, 2 & 4 of UR200 when play with the magnet. Just in case, replace UR200.