Do you mean the gear selector switch? The one that shows on the display on the dash?
I mean the black box with wires (aka auxilliary water-butt) on the back of the gearbox that senses what gear you are in so the computer can tell the dash what to display, including neutral, as that doesn't work either.
I guess if it's a quick job, either it can be taken off without removing the swing-arm, or the latter comes off easily!