Keyboard backlight not working for some keys Many similar questions I've read here have to do with backlights not working at all or when dimmed.

My problem is that the number 6 key is not illuminating. All other keys are working as they should.

Any ideas how to diagnose and fix this?
Unfortunately, unless a SMC reset doesn't work (Shut down -> Press Shift-Control-Option and Power Button at the same time for 10sec -> release keys -> Press power button), it is a hardware problem. If it's under warranty, Apple should fix it, if not, a 3rd party repair shop should be able to fix it for a reasonable price.