As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE ALTITUDE OF FL200 MACH NR APPEARED. SELECTED C/O BUTTON ON AUTOPILOT PANEL AND THE BUTTON REMAINED IN THE DEPRESSED POSITION. NO OTHER MODES WERE ALLOWED TO BE SELECTED AND AUTO PILOT WOULD NOT RE ENGAGE. REMOVED AND REPLACED FLIGHT CONTROLLER GC-810 OPPS CHECKS GOOD, REF MM CH 22-10-03.