As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE ON FLIGHT, WITH THE AUTOMATIC PRESSURIZATION ALREADY DEFERRED, THE CREW RECEIVED AN EXCESSIVE CABIN ALTITUDE WARNING AND CONTINUOUS CHIME. AFTER CONSULTING THE QRH, THE CREW DECLARED AN EMERGENCY AND RETURNED TO INDIANAPOLIS WITHOUT FURTHER INCIDENT. MAC MAINTENANCE PERFORMED A SUCCESSFUL OPERATIONAL CHECK OF THE MANUAL PRESSURIZATION CONTROL.