What was found
FLT 6506 - IAD-BOS - RETURNED TO IAD DUE TO PRESSURIZATION INOP IN AUTO MANUAL, CABIN HIGH ALTITUDE AND CABIN OXYGEN LIGHT ILLUMINATED, LEFT AND RIGHT O/T DUCT LIGHT ILLUMINATED AND NO CONTROL OVER CABIN TEMP IN AUTO. MAINTENANCE FROM 1 AND 2 ABOVE OPENED DMI NR 21790 IAW MEL 21-2 ON THE PRESSURIZATION SYSTEM. DMI NR 21790 WAS LATER CLEARED AT LYH BY CLEANING OUTFLOW VALVES SCREEN FILTERS, OPS CHECKED GOOD, CAN NOT DUPLICATE ON GROUND. FOR 3 AND 4 ABOVE PERFORMED OPS CHECK IN BOTH AUTO AND MANUAL WITH BOTH BLEEDS ON FULL BUT COULD FIND NO DEFECTS. (Z)