What was found
LEVELING AT 23,000 FT, A HISSING NOISE STARTED. REFERENCED PRESSURIZATION PANEL, DIFF GAUGE SHOWED NEEDLE ON RED LINE NEAR 6 PSI, RATE GAUGE SHOWED + 500 FT/MIN, ALTIMETER GAUGE AT 6000 FT. PRESSORIZATION SYSTEM IN AUTO THE WHOLE FLIGHT, DIFFERENTIAL PRESSURE AT 6 PSI FOR APPROXIMATELY 45 MINUTES. DECLARED AN EMERGENCY. DIVERTED, AND LANDING UNEVENTFUL. MAINTENANCE REPORTED AFT OUTFLOW VALVE FAILED. FAULT CODE IN CDS. COMPLIED WITH FUNCTIONAL TEST OF THE PRESSURIZITION CONTROL IAW AMM 21-31-00-720-801. TESTED GOOD & FAULT CODE WAS NOT PRESENT. REMOVED AND REPLACED THE AFT OUTFLOW VALVE IAW AMM 21-31-16-000-801, & 21-31-16-400-801, OPS TESTED GOOD. RETURNED TO SERVICE.