As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - FLT 528 - AFTER TAKEOFF, AIRCRAFT FAILED TO PRESSURIZE. AIRCRAFT RETURNED TO ORD AND LANDED WITHOUT INCIDENT. REPLACED NR 1 CABIN AIR PRESSURE CONTROLLER AND ALSO REPLACED STATIC OUTFLOW VALVE FILTER. SYSTEM GROUND CHECKED NORMAL OPERATION. (M)