As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CLT/ORD - FLT 1527 - AFTER TAKEOFF, THE CREW WAS UNABLE TO PRESSURIZE THE AIRCRAFT IN EITHER THE AUTO OR MANUAL MODE. THE AIRCRAFT RETURNED TO THE FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE LEFT AFT SERVICE DOOR MECHANISM OUT OF SEQUENCE AND NOT SEALED. THE DOOR WAS CYCLED SEVERAL TIMES, ALL OPERATIONS CHECKED NORMAL. (X)