As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - FLT 136 - CREW WAS UNABLE TO PRESSURIZE AIRCRAFT UPON CLIMB-OUT FROM DEN. FLIGHT RETURNED TO FIELD AT DEN AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE LEFT PACK FLOW CONTROL VALVE. OPERATIONAL CHECK GOOD. (M)