Jeepfreak81

Hello all, my dad and I are working on a 3500HD cab and chassis dump truck he recently picked up. We're having issues with a trouble code for a high reading on fuel circuit B (forget the actual code number). This truck has had the rear tank removed and we did find a 121 ohm resistor plugged into the sending unit harness for the rear tank. We checked it and it does read 121 ohms. We checked the voltage at that plug and it's only reading .5 volts. The front tank which has a 3 wire sending unit for some reason reads 5 volts. Surely this isn't correct. Any ideas on where and what to look at next?