As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL290, CREW REPORTED THE CABIN ALTITUDE WARNING HORN SOUNDED. CABIN WAS ABOVE 10,000 FT. CREW FOLLOWED QRH PROCEDURES AND CABIN KEPT CLIMBING. AN EMERGENCY DESCENT WAS INITIATED TO 10,000 FT. PRESSURIZATION WAS STILL ERRATIC IN MANUAL MODE. FLIGHT RETURNED TO FIELD AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE PERFORMED A/C PRESSURIZATION T/S. NO FAULTS FOUND. REPLACED CABIN PRESSURE CONTROLLER AS PRECAUTIONARY PER AMM 21-31-00-702-001. CHECKS OK.