As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AUTO PRESSURIZATION IN FLIGHT FAILED. CREW WAS ABLE TO MAINTAIN CABIN PRESSURIZATION THRU MANUAL AC. FLIGHT RETURNED TO CLT. AT FL280, CABIN BEGAN TO CLIMB TO 12,000 FEET & GOT WARNING HORN. PAX O2 MASKS DID NOT DROP. FLIGHT LANDED WITHOUT FURTHER INCIDENT. R & R AFT OUTFLOW VLV IAW AMM 21-31-11-404. OPERATIONS CHECKED GOOD. PERFORMED SATISFACTORY CABIN PRESS LEAK TEST IAW AMM 05-51-91-702.