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i got a 15" MBP (820-3332) with no display an no backlight. PPVCORE_GPU looks fine (1.06V) but LCD_PWR_EN is missing (no short to ground). I got USB activity but no image on external display.
Could it be a faulty GPU?
Missing internal and external image at same time, points to graphics issue.
Try PRAM reset 2-3 times first.
And resolder U8900 with leaded solder, as usual in this board.