As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN ALTITUDE WARNING HORN CAME ON. CABIN ALTITUDE WAS 10,000 - DIFF ABOUT 7.0. WITH OFF SCHEDULE DESCENT OUT LIGHT ON. NO MASTER CAUTION. DESCENDED LOWER ALTITUDE AND AT FL210, CABIN RETURNED TO NORMAL. FOLLOWED QRH. IN MANUAL AC, OUTFLOW VALVE WOULD NOT MOVE - WAS SHOWING MOSTLY CLOSED. PACKS IN HIGH SEEMED TO HOLD A BIT. MX R & R AFT OUTFLOW VALVE IAW AMM IAW AMM 21-31-11-004-043-01. OPS CHECK GOOD IAW AMM 21-31-00-702-001. ALSO, R & R CABIN PRESSURE CONTROLLER IAW AMM 21-31-21-404-004. OPS CHECK GOOD IAW AMM 21-31-00-702-001.