Id say 99 % of the timeits the sender.Almost every fuel gauge problem is there.Mostly are bad floats and second is the sender itself.Easy to check.Inhook the wire to sender.Attach a ggod ground to the wire and turn on key if gauge moves its in the sender.Ifnot as said check t see if it pulses to sender.The other big problem is the ground for the sender.many have rusted awy and lots have lost continuity.Oh yeah if your car get a service manual.Rocky