Hello all,
I just picked up my C172 from annual maintenance. On the flight home I discovered that my S-TEC 55X autopilot does not accept any mode despite a „RDY“ on the display which indicates a successful self test.
Any ideas?
55X in a 172? Cool! But wait, don‘t you always run the full 70-item preflight checklist? 
1970 FR172
The pre-flight checklist for the 55x is crazy indeed. I did it exactly once!
However no chapter for troubleshooting or non-normal behaviour whatsoever.
RDY starts blinking or display goes blank when trying to select a mode.
I would check the connectors. Often, during a service, a cable harness gets unplugged and since a “mechanical” service does not involve avionics, none of that gets checked. I once started to taxi after some major work, away from base, and (as it was getting dark) found the instrument lighting to be dud. A connector had become unplugged. Had I departed a little sooner I would not have picked this up and would have had a load of fun flying with invisible instruments 
Thanks Peter!
I do agree!
There is no reason why the unit should fail overnight, especially since no one worked on it.
It might get a false permanent A/P disconnect signal due to a loose cable…
