What was found
ON INITIAL DESCENT DIALED ALT FROM FL360 TO FL240. ALT WINDOW CHANGED TO FL500 ALONG WITH AURAL/WARNING. CYCLED BOTH FLT DIRECTORS MULTIPLE TIMES BEFORE WINDOW WOULD RESET TO DESIRED ALTITUDE. GROUNDED - YES. PERFORMED DFCS BITE FAULT REVIEW, FOUND MSG "MCP ALT REF MON INV" 22-11066/22-11067 FOR CHANNEL A+B IN FAULT HISTORY. RAN MCP TESTS IAW B737-900 FIM 22-11 TASK 803, COULD NOT DUPLICATE DEFECT, ALL TESTS PASS. PERFORMED LAND VERIFY IAW B737-900 AMM 22-11-00-700-801, ALL TESTS PASS. A/C OK FOR SERVICE. APPROACH SURVEY COMPLETE.