I´ll try to get your RE-ARM Board (+ Ramps 1.6) with a 3dtouch sensor to work. It nearly always say M999 after / during homing Z axis. The 3Dtouch has an own power supply (calibrated on 5,0 V). I´ve also tried to add a 10kOhm resistor between GROUND and Z-MIN. In 4 of 5 cases I see a M999, in 1 case get a finished homing procedure.