What was found
AT FL390 APPROXIMATELY 20 MINUTES AFTER LEVEL OFF RECEIVED CABIN ALTITUDE WARNING. CABIN ALTITUDE READ 10,000 AND SLOWLY INCREASED TO APPROXIMATELY 12,000 . OXYGEN MASKS WERE DONNED. CABIN ALTITUDE REMAINED ABOVE 10,000 UNTIL DESCENDING THRU APPROXIMATELY 15,000 . FOUND CABIN MODE SELECTED TO # 1, REMOVED AND REPLACED # 1 AUTOMATIC PRESSURE CONTROLLER REF AMM 21-31-02-4. OPS CHECKS GOOD GOOD. ALSO PERFORMED OPS CHECKS OF # 2 AUTOMATIC PRESSURE CONTROLLER ALSO CHECKS GOOD REF AMM 21-31-02-4. AC OK FOR SERVICE