What was found
CABIN ALT WARNING AT F270. CABIN ALT INDICATE 13K FT. COMPLIED W/ ACTION ITEMS & QRH. 20 MIN OUT AT 27,000FT GOT WARNING HORN, ALTITTUDE INDICATOR SHOWED 13,000 FT, CREW BROUGHT ACFT DOWN & STABILIZED AT 13,000 FT, BEGAN DESCENT & LANDED. PILOTS USED BOTH OXYGEN MASKS & CREW OXYGEN DOWN TO 1,000PSI. PERFORMED A CABIN PRESSURE CONTROLLER BITE IAW FIM 21-31 TASK 801, BITE TEST OF CONTROLLER 1 SHOWED 4 FAULTS ON THE LAST LEG, CONTRL LRU FAIL CODE 042, MANUAL SWITCH CODE 058, CAB ALT 10000 FT CODE 017, & LO. ACCOMPLISHED PRESSURE LEAK DOWN CHECK IAW AMM 21-00-05-780-801. LEAK DOWN FAILED. PERFORMED FIM 21-31 TASK 824, PERFORMED A PRESSURE SYS GRND TEST IAW AMM 21-31-00-700-802. SYS TEST PASSED. REPLACED OUTFLOW VALVE. TESTED OK. MASKS IN CABIN WERE DEPLOYED, FROM FLIGHT DECK, & GENS UNPINNED. INFL/ HI LEAKING CODE 30, CODES 058 & 017 SHOW UNUSED IAW FIM 21-31 TASK 801.