What was found
UNABLE TO MAINTAIN CABIN ALTITUDE, AT 36,000 CRUISE FLIGHT THE CABIN HORN CAME ON. THE CABIN CLIMBED TO 11,000 FT, MASKS DID NOT DROP, RAN QRH. DESCENDED TO 10,000 FT. PRESSURIZATION SYSTEM RAN NORMAL AT 10,000 FT. REMOVED AND REPLACED PRESSURE CONTROL MODULE PER AMM 21-31-25. PERFORMED LINE MX PRESSURIZATION HEALTH CHECK PER AMM 21-30-00, AIRCRAFT PASSED, OK FOR SERVICE.