Is the drive unit still turning/ driving, and is the turn always in the same direction. A rudder
not moving will almost always result in a turn one way or another.
Might want to disconnect the drive unit and connect it directly to a fused source of 12/ 24 volts and see if it will drive both directions (reverse polarity). Also the rudder
position sensor is a wear unit. If you can operate a voltmeter might want to monitor
the RP voltage at the connection to the AP control unit. Basically + fixed voltage (red wire), negative (black wire), and position signal ( blue or green but don't quote me on that). Should have a smoothly changing voltage between the signal and ground as the wheel