As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EN ROUTE FROM PIT TO IND, AT FL200, THE CABIN ALTITUDE WARNING HORN SOUNDED WITH NO AUTO FAIL LIGHT. CREW DESCENDED FLIGHT TO 12,000 FEET AND USED MANUAL AC TO CLOSE OUTFLOW VALVE BUT COULD NOT OPEN IT. CREW HAD TO USE MANUAL DC TO OPEN OUTFLOW VALVE. FLIGHT CONTINUED ONTO IND AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE AFT OUTFLOW VALVE PER MM 21-31-11-400. OPS CHECK GOOD PER MM 21-31-00-1201. (M)