What was found
PROMPTLY AFTER LEVEL OFF AT FL 310 HEARD CABIN ALTITUDE WARNING HORN ACCOMPANIED BY VISUAL CABIN RED ANNUNCIATER LIGHT PERFORMED QRH CHECKLIST PROCEDURES & EXECUTED EMERGENCY DECENT TO 10000'. CABIN STABILIZED IN STANDBY MODE AT 10000 REMAINED OF FLIGHT. ACFT GROUNDED. PERFORMED OPS TEST, PRESSURIZATION CONTROL SYS IAW AMM 21-31-00, OPS CHECKS GOOD, NO FAULTS FOUND. R & R AUTO CABIN PRESSURE CONTROLLER FOR CONVENIENCE, PRECAUTION. PERFORMED OPS TEST, PRESSURIZATION CONTROL SYS AFTER INSTALLING NEW AUTO PRESSURE CONTROLLER, NO FAULTS FOUND, REF AMM 21-31-00. PERFORMED AIRPLANE SINGLE PACK OPERATION, CONFIDENCE CHECK IAW AMM 21-00-05, NO FAULTS FOUND, ALL TESTS PASS.