An open circuit in the wires to the guage, or an "open spot" in the windings on the sender....

gil in Tucson
 
It's been forever since I installed mine, but I seem to remember seeing a post on here once about installing them upside down.

Steve Zicree
 
Stuck ?

When I permanently mounted the wings and attached the guage wires, I was surprised to see that they read full even though there was no fuel in either tank! I thought i had them upside down ... so I removed one of the covers to check and it was indeed correct. I replaced the cover and the guage then read correctly. I realized that with all the time the wings spent in the cradle, the float arms had become a bit "stuck" in the up position. I then just tapped the cover plate of the other tank with a screwdriver, and the arm fell in to position.