I searched the Configuration.h and Configuration_adv.h of the stock fw, but didn’t find anything about the filament runout sensor pin yet.
However, looking at the pic of the mobo, it seems that there is a free 3pinned connector which is blue and at the backside it’s labeled as 3.3V/S/GND. I know the mobo (and the processor being used there) of the regular Neo isn’t the same, but there this exact connector is given as well and it’s being used for the photoelectric z endstop.
I’m actually wondering if here at the K2Neo this connector is connected to the processor as well - I’d assume it is and AC just didn’t bother to add an extra physical z limit switch. Means, it might be possible to add a limit switch here if we could find the belonging pin assignment.
At the Neo this pin is endstop_pin: ^PC14, and PC14 doesn’t seem to be used in the cfg of the K2Neo. Might be worth a try (again: yes, mobo and processor aren’t the same…) - but as I don’t have the K2Neo, I can’t test anything myself, so this is NOT a recommendation to do/try it as I don’t wanna be made responsible if anything breaks… 