As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK / DECLARED EMERGENCY. ON CLIMB PASSING FL 240 CABIN ALT WARN HORN SOUNDED. CABIN ALT CONTINUED DESCENT TO LANDING AT 500FT / MIN IN AUTO MAX CABIN ALT 10" STABILIZED AT 3" IN AUTO. REMOVED AND REPLACED OUTFLOW VALVE AND HEATER GASKET PER MM.