As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IAD - CREW REPORTED UNABLE TO PRESSURIZE CABIN. EMERGENCY DECLARED FLIGHT RETURNED IAD LANDED WITHOUT INCIDENT. PLACARD NUMBER 2 CABIN PRESSURE CONTROLLER PER MEL. REPLACED NUMBER 2 PRESSURE CONTROLLER AT DFW ON 07/23/2011. SYSTEM GROUND CHECK NORMAL OPERATION.