As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHX - FLT NR 580 - AIR TURN BACK, CREW UNABLE TO PRESSURIZE AIRCRAFT DURING CLIMB-OUT. AIRCRAFT RETURNED TO PHX AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE LEFT AND RIGHT PACK VALVES. OPS CHECK OK. AIRCRAFT RETURNED TO SERVICE. REF NR AWXA9500664. (M)