As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB AT FL210, CABIN PRESSURIZATION STARTED A SLOW UNCONTROLLABLE ASCENT. UPON CABIN ALTITUDE REACHING 10,000 FEET, INITIATED AIRCRAFT DESCENT AND RETURNED TO PHX WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED RIGHT OUTFLOW VALVE. OPERATIONAL CHECK NORMAL.