in order to verify the integrity of your gauge and wiring unlpug the lead form the sender on the tank if you ground the lead it should show empty if i recall and if you unplug it gauge should show full if gauge does not move when you do this fault is likely in the gauge but to be safe check the ground continuity and continuity of the signal lead from the sender to the gauge. if the guage moves when you do this it would indicate a faulty sender in the tank