As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED, AFTER TAKEOFF WERE UNABLE TO CONTROL THE CABIN ALTITUDE IN MANUAL MODE WHILE THE AUTO MODE WAS ON DEFERRAL. IT WAS REPORTED THAT THE CABIN ALTITUDE WAS CLIMBING AT APPROXIMATELY THE SAME RATE AS THE AIRCRAFT AT ALL SETTINGS. THE AIRCRAFT RETURNED TO KORD AND LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED THE DIGITAL PRESSURE CONTROLLER TO CLEAR THE DEFERRAL ON THE AUTOMATIC SYSTEM, AND THE MANUAL PRESSURIZATION MODE OPERATED NORMALLY WHEN TESTED. THE AIRCRAFT WAS RELEASED FOR SERVICE WITH NO FURTHER DEFECTS NOTED.