If this is a wireless unit, make sure your sensor batteries are fresh and properly installed. If it is a wired unit, make sure the wires are undamaged and properly attached. Since the unit is a few years old, it is possible that you have oxidized contacts somewhere that just need a little cleaning. I have two Bontrager W4 wireless computers and on one of them the fork sensor has gone bad twice. LBS replaced free of charge both times.