As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CLT - CLIMBING THROUGH 21,000 FEET, CREW REPORTED THE CABIN ALTITUDE WARNING HORN SOUNDING. CABIN REACHED 13,000 FEET. PRESSURIZATION WOULD NOT REVERT TO STANDBY. CREW DECLARED AN EMERGENCY AND RETURNED TO FIELD AT CLT. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE CABIN PRESSURE CONTROL PANEL (IN OVERHEAD) PER MM 21-31-25. PERFORMED OPERATIONAL TEST OF PRESSURIZATION SYSTEM PER MM 21-31-00. CHECKED OK.