As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF THE CREW REPORTED THAT THE AIRCRAFT DID NOT SHOW EVIDENCE OF PRESSURIZING. THE CABIN RATE FOLLOWED THE AIRCRAFT'S RATE FOR THE FLIGHT'S DURATION, AS DID CABIN/AIRCRAFT ALTITUDE. THE CREW REPORTED THAT WITH PACK 1 ON MEL, PACK 2 HAD INSUFFICIENT AIRFLOW TO PRESSURIZE THE AIRCRAFT. THE FLIGHT RETURNED TO MKE AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND A DUCT THAT HAD DISCONNECTED FROM THE ACM. THE DUCT WAS RECONNECTED AND THE PRESSURIZATION SYSTEM OPERATED NORMALLY.