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This A2251 MacBook Pro with mild liquid damage (USB-C area) is working fine on external display but does not recognize the internal LCD. I tried several displays and flex cables, and there is no backlight and no display. The internal does not show up in Settings>Displays, only the camera is...

Never mind. I just found a short in the CPU (DP_INT_AUXCH_C_N). I'm trashing this board.

Correct about my tester screen: it was either the wrong revision or malfunctioning. Connected a known good 2019 display and the keyboard brightness option is back in Settings. However, still no backlight is produced. The output (PPVOUT_S0_KBDBKLT_R) starts at 5.1V, then briefly jumps to 29V...

This board from an Air A2179 arrived with liquid damage and PPVOUT_S0_LCDBKLT shorted to ground. After replacing several corroded capacitors, the short was removed, and the machine started up with the LCD backlight fully functional. Initially, the keyboard backlight appeared functional as well...

Ok, with all peripherals connected, the DFU Restore went through to the padlock. But no video on the target machine. No startup. Assume bad CPU?

No, just the logic board, USB, and battery. Is that really necessary?

Data is irrelevant so I did a Restore. Note that I had a hard time getting the machine into DFU mode, so I used a blank SoC ROM chip to force DFU. I have: PP3V3_G3S: 3.2V PP1V8_G3S: 1.8V PP0V9_SSD: 0.9V PP1V8_SSD: 1.8V PP2V7_NAND: 2.5V

Yes, PP5V_G3S is stable now. DFU Revive ends in error -1.

I just placed all components back on the board and it looks like PP5V_G3S is at 5.1V, even with the short. No startup, though.

No, an original battery pulled from another A2179 topcase.

Diode mode red-to-ground: 0.246, black-to-ground: 0.250 Resistance: 305 Ω

Battery has been replaced.

After lifting every capacitor on this rail, no luck. It will remain a mystery.

This 2020 MacBook Air (820-01958) shows no signs of damage other than a bad battery, which I replaced. It loads Internet Recovery very slowly and, after 20 minutes, always ends with error -2003F. It fails to enter DFU mode (both keyboard method and using FORCE_DFU pins). So I removed and...

No thermal imaging here, sorry. I just replaced U8110 and my short changed from 100 to 400 Ω. Doesn't make a lot of sense to me. Could there be a short inside the board that reacts to the temperature changes during soldering?

Yes, I have both CD3215s off the board again. Yes, when I lift L7670, the short is on pad 1 but not on pad 2. Yes, I tried lifting all resistors and inductors (one by one) that connect from PP5V_G3S to subnets: L8570, L6703, R7100, R8103, R8144, R8147, R8444, and R8445. And I lifted U7210/30/90...

This 820-01521 2019 Air board had liquid damage in several places, was missing PPBUS, and had shorts on both CD3215s. I replaced all corroded components I could find, including both CD3215s. Now I'm left with PP5V_G3S which shows 80-100 Ω to ground (0.07 in reverse diode mode). No shorts on any...

After carefully looking at the liquid damaged area, I found the culprit. There was some invisible corrosion on the via for the powers supply to U8400 (PP5V_S0_BKLT_D). It was only delivering 4V instead of 5.1V. Bridged with a jumper wire and backlight is back in full strength.

All resistors check out correctly (I replaced almost all since they were liquid damaged). What's your guess as to what causes the backlight to disappear when brightness is increased?

Yes, of course. All 3 assemblies I tested show the same behavior.