If I remember it would be difficult to cross up the electrical plug since it can only plug one way. It's more likely to me if there was a wiring cross up it would be inside the cap at the 3 plugs. Or, it could be a mechanical issue where the float of one of the sending units is jammed. Either way sucks becuase pulling the tank is a pain. Attached are a couple of files from the manual that may give more insight. I will say that when my sending units were bad the gauge read empty. I replaced both of them and the pump while I had the tank out.