Probably looks something like this. Moeller used to make these. Careful when you yank it out that you don't damage something. You want to carefully extract
USUALLY (though not always), the failure to read is a function of the GROUND not being correctly set-up to the tank and sensor. -- especially in the remote read-outs that are added in the cockpit.
Also, though its probably unnecessary to say, remember: The vapors in the tank are flammable -- no sparks checking the circuit to the open tank near the opening :^))) Use you meter carefully.