As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LAS/BUR - PRESSURIZATION WAS INADEQUATE DURING INITIAL CLIMB. FLIGHT WAS ABORTED. AIRCRAFT RETURNED TO LAS LANDING WITHOUT INCIDENT. MAINTENANCE REMOVED AND REPLACED PACK FLOW CONTROL VALVE AND THE LEFT AIR CYCLE MACHINE PER BMM CHAPTER 21. BOTH OPS CHECK GOOD. AIRCRAFT RETURNED TO SERVICE. REFERENCE AWXA9500795.