As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN PRESSURE WARNING, RAN APPROPRIATE CHECK LISTS AND EXTINGUISHED AT 1000'. WARNING OCCURRED ON FLT 2821 AS WELL, SO HAD BEEN MONITORING. HELD PRESSURE THROUGH FL180, THEN AS LEVELING AT FL 220 THE CABIN PRESSURE KEPT CLIMBING ABOVE 9000'. STARTED A DECENT AND LIGHT CAME ON AND CABIN PRESSURE REACHED 11,000' BEFORE DECREASING. RETURNED TO FIELD. LANDING UNEVENTFUL. R & R AFT OUTFLOW VALVE REF AMM 21-31-16-000-801 AND 21-31-16-420-001. ACCOMPLISHED FUNCTIONAL TEST OF THE PRESSURIZATION CONTROL REF AMM 21-31-00-720-801. RETURNED TO SERVICE.