Fn, in a normal key tester
Fn/layer keys, lighting controls, etc. are unable to be detected using a key tester. That's because a key tester can only detect keycodes that a keyboard sends to the computer, whereas for these special codes, they are handled internally by the keyboard firmware and aren't sent to the computer.
Fn/layer key, you'll need to use one hand to hold that key down with your tool, while connecting the switch pin pads together for another key that uses the
Fn/layer key. Then see if the key tester will detect a key press. For example, on some PCBs, holding down
F1to a computer.
KEY TESTERtab and then selecting
Test Matrix. You will then see the special keys light up in the app when pressed.