As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON DESCENT CLEARANCE FROM 270 TO 13000 WHEN WE DIALED 13000 IN THE ALTITUDE WINDOW IT SHOWED 50000 AND WOULD NOT CHANGE UNTIL WE DESELECTED A/P AND CYCLED F/G'S ON AND OFF. GROUNDED - YES. TASK CARD COMPLIED WITH REMOVED AND REPLACED DFCS MODE CONTROL PANEL PER B737-900 AMM 22-11-34. GROUND VERIFICATION TESTS PASSED.