I just fixed mine on my 521vx. First remove the seats. To do that you remove the center fusion on the two seat backs and there are two screws behind each one. Then slide the seat forward slightly and lifting up and out of the way. You will now see the fuel tanks and the sending unit on each one. Take one wire off of the sender and touch it to the other with the power on and touchpad on each side when you touch them. If the gauge does not move when the wires are touched then the gauge is bad. If it does move the your sending units are bad. Both of my sending units were bad, and I replaced them with mueller units that were a direct fit and about $30 each.
2003 Ranger 521VX 35th Anniversary 250 Yamaha
1979 Ranger 168V 140 Evinrude - Papa Smurf