that is a shuttle lever code. try re calibrating the hot transmission.
try to calibrate the transmission, but put the shuttle lever in reverse instead of forward and see if error U 23 comes up (shuttle lever not in forward) then shut off tractor go to calibrate again but lever shuttle lever in neural, see of U 23 comes up.
it could be in the shuttle lever,
or in the forward / reverse position switches on top of the transmission, under the cab floor