If the gage reads over full all the time there is most likely an open wire between the gage and the sending unit in the tank or the sending unit itself is bad. If it reads empty all the time it could be a short to ground in the wire between the gage and the tank sending unit or a shorted sending unit, could be the float is stuck, or there is no power to the gage. Try disconnecting the sender wire back at the tank and see if the gage reads over full. If it does then the problem is in the tank. If it still reads empty then the problem is a shorted wire between the gage and the pump or there is no power to the gage.