What was found
DURING DESCENT, IN LNAV/VNAV PATH, AIRCRAFT DESCENDED PREMATURELY AT A FIX WITH AN ALTITUDE RESTRICTION OF 14,000', CROSSED THE FIX APPROXIMATELY 150' LOW. THEN AGGRESSIVELY PITCHED NOSE DOWN AND DID NOT CAPTURE THE NEXT ALTITUDE THAT WAS SET IN THE MCP, 13,000'. DISCONNECTED A/P A AND LEVELED OFF. DID NOT TRY A/P B. ACFT GROUNDED. ACCOMPLISHED DFCS BITE IAW FIM 22-11-00-810-801. NO CURRENT FAULTS, LEG 1 SHOWS NO FAULTS, LEG 2 SHOWS MESSAGE 22-21700 CH A, COLUMN OVERRIDE. THIS MESSAGE CONFIRMS CAPTAIN'S DESCRIPTION OF CHANNEL A DISCONNECT MANUALLY BY FLIGHT CREW. UNABLE TO DUPLICATE FAULTS IAW LWMP SYSTEM, TABLE 22-00-01-730-801, AND HGS 4000, SMM 34-36-00. LAND VERIFY AND HGS SYSTEM TEST ACCOMPLISHED. ALL REQUIRED TESTS PASS.