Chosen Solution
I have a Macbook Air with brightness that doesn’t change, even though I slide the brightness up and down in the Display preferences. I also tried with the keyboard keys. The brightness icon appears and the gauge does change, but the actual brightness of the screen never responds. What could possibly be wrong here? Logic board, inverter board, the LCD?
Most likely an issue with BKL_PWM not changing on the logic board (this adjusts brightness). Check R7704 resistance and check for a short on that line (C7704 or U7701 can do this). If you replace U7701 and still have the same issue, the signal to adjust brightness comes from the PCH so there is not much you can do.
