As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK / DECLARED EMERGENCY. FLT 220 CRUISE CABIN ALT WARNING HORN SOUNDED. CABIN ALTS CLIMBING AT 700FT/MIN. PERFORMED CABIN ALT WARN/ABNORMAL PRESS CHECK PER QRH AND INITIATED DESCENT COULDN'T GET CABIN TO DESCEND ON NORMAL PRESS. CONTROLLED CABIN USING MAIN DC. REMOVED AND REPLACED OUTFLOW VALVE ASSY PER MM.