As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - CREW REPORTED UNABLE TO PRESSURIZE AIRCRAFT AFTER TAKEOFF. THE OUTFLOW VALVE WAS CLOSED AND LEFT PACK SHOWED 50 ON THE PRESSURE GAUGE. RIGHT PACK WAS ON A PLACARD INOPERATIVE. EMERGENCY DECLARED FLIGHT RETURNED ORD LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. FOUND AIR PRESSURE SENSE LINE ON LEFT PACK LOOSE. RE-TIGHTENED AND TORQUED SENSE LINE. SYSTEM GROUND CHECK NORMAL OPERATION.