Function Key Not Working MacBook Air

New Contributor II

This is the second MacBook Air Early 2015 where the function keys on the keyboard stop working. When you press the buttons, you hear a 'dunk' sound indicating there's input. When I boot to a good known OS, the same thing. I have tried reseting various plist, replaced the top case, re-loaded OS, etc... Has anyone else encountered this? I'm still convinced its a software issue but I've tried everything I know.