What was found
CABIN ALTITUDE WARNING 1.5 HRS FROM LANDING. FOLLOWING EICAS MSGS, CABIN ALTITUDE, AMBER CABIN AUTO INOPERATIVE. CREW UNABLE TO CONTROL PRESSURIZATION IN AUTO OR MANUAL MODES. CREW EXECUTED THE APPROPRIATE CHECKLIST, DECLARED EMER, DESCENDED & COMPLIED WITH A COURSE DEVIATION IAW QRH PROCEDURES. UPON INTEGRITY CHECKS OF SYS 21, OVERBOARD EXHAUST VLV ACTUATOR FAILED TO OPERATE. VLV CEASED TO FUNCTION BETWEEN THE NORMAL & SMOKE POSITIONS. OVERBOARD EXHAUST VLV WAS R & R. EQUIPMENT COOLING TEST & OPERATIONAL CHECKED SATISFACTORY IAW AMM 21-58-05. FAULT NOT DUPLICATED. NO ADDITIONAL FAULTS NOTED. ACFT VERIFIED RELEASED TO SERVICE ETOPS 180.