What was found
CLT - AT FL330, EN ROUTE FROM CLT TO PVD, THE CABIN ALTITUDE WARNING HORN SOUNDED. DESCENDED TO 10,000 FEET AND RETURNED TO FIELD AT CLT. AUTO, STANDBY AND MANUAL PRESSURIZATION MODES WERE INEFFECTIVE. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE AFT OUTFLOW VALVE PER MM 21-31-11-404-042-001. OPERATIONAL CHECK AND PRESSURIZATION CHECK GOOD.