I recently received a pre-assembled E3D-V6 hot end to swap in for my Hexagon.
I connected the thermistor and heater wires exactly as for the hexagon (thermistor to T0, heater wires to D10) and switched on the printer. The initial temperature seemed accurate (20 degreesC), but instantly the temperature reading was climbing up, despite the fact the 'target temperature' was 0. Indeed the little red LED on the RAMPS board to indicate power was on, i.e. the hot end was spontaneously heating itself. I switched off and disconnected the heater wires and switched on again with only the thermistor connected. Now D10 read ~0V, no red LED.
Probably unrelated but I noticed that when I gently touched the little screw which held the thermistor wires against the heater block, I'd get spurious temperature readings (~700 degreesC), so I loosened that screw a touch as I thought the screw was currently at risk of pressing through the wire insulation.
Anyway, the exact same issue returned when I plugged back in the heater wires; spontaneous heating. I returned to the Hexagon and everything works fine again.
Any ideas what could cause this behaviour in the E3D-V6? Possibly a short between the thermistor and heater wire?
[Running RAMPS 1.4 & Arduino MEGA 2560]