There was an uncharacteristic humming sound. The clutch would engage, then there was a barely audible soft humming sound not ordinary to the installation, then a fault. Usually within a second or two. I can back drive the motor by forcing the wheel, I did this while engaging the pilot on exactly the suspicion than perhaps the commutator was dirty but there was no change. I tried measuring the peak current using a clamp ammeter. I measured the current with the motor connected directly to power to verify no true high current fault. I de-installed and re-installed the course computer completely. Always the same fault. The rudder angle sensor (you are right it is a rotary) always indicated correctly. I re flashed the autopilot firmware and recommissioned it. No change. This was all while motoring down the Ditch. We had left Little Creek in Norfolk intending to turn right and go around Hatteras, but as the autopilot failed immediately I turned left instead and went for the canal. Had an ebay seller overnight their used one to me in Roanoke, installed it (just the course computer) and it worked perfectly to the end of the Exumas and back to Florida. Guess I should try it here on the west coast .

The Raymarine technician working on the broken (no problem found) one did say that the error message is very general and catches a lot of stuff.

Mine had worked on the trip down the Chesapeake in December after the year layup, no problem. I had checked it a couple of times at the dock. There is a possibility that some condensation got into it in Norfolk, there were a couple of days when everything sweated like crazy.