Can’t turn on youror Air’s keyboard backlighting? No it’s probably not broken, it’s most likely the light sensor. If the ambient light sensor on MacBook Pro and models detects too much light, the keyboard illumination will disable itself and refuse to turn on, showing the above locked image instead.
You can force the keyboard illumination to turn back on by covering up this ambient light sensor, which is located directly to the left of the iSight/FaceTime camera on MacBook Pro and MacBook Air models:
If you look carefully at that area on the screens border, you’ll see tiny perforations where light is detected. Covering the sensor will enable the backlit keys to turn on, usually instantly, and you can then adjust the backlight as usual with the F5 and F6 keys.
Why does this happen at ll? It’s actually a power saving feature. Your MacBook determines that if it’s bright enough, the glowing keys aren’t needed, so it disables them to maximize battery life. Now you know.