As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MCP ALTITUDE SWITCH TO 50000 FT AMBER BOXES AROUND CDU ALTITUDES BOTH SIDES FLASHING INTERMITTENTLY, HORN GOING CONTINUOUSLY. ON GROUND MCP ALTITUDE WENT TO 0000. ACFT GROUNDED. FIM TASK 22-11 TASK 803. PERFORMED BITE TEST OF A & B CHANNELS FOR MCP, NO FAULTS FOUND AT THIS TIME. ALL TESTS PASSED.