If the "incorrect gauge" fix from Rich Cutler doesn't work, I would try the following:
- remove the left-side rubber grip (fixed with double-sided adhesive)
- remove the 3 small screws holding the metal plate under this grip in place
- check if the ribbon cable coming from the top plate is still in place in its plug under the plate.
This cable connects the body to the gauge. The plug can be seen on this picture:
It is the grey square in the middle of the left part of the body, partly covered by the ribbon cable coming from the speed dial.
If it is unplugged or loose, put the gauge ribbon cable back in place. On the same picture, it is the second ribbon cable from the left going out of the top plate.
You have to pull up the blocking slider around the plug (the brown part on the picture between the plug and the blue cable). Use a fine pair of tweezers, or a small and hard pen (in this case, one side after the other).
Just put the cable deep into the plug, and push down the brown slider do fix it.
You don't need to remove completely the top plate for this manipulation, just the metal plate, which is quite easy. I can imagine that vibrations or a small shock could put this cable out of place. I hope you can successfully fix it!