As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 283 LAS-LGB, AIR RETURN TO LAS - ON CLIMBOUT THE CREW LEVELED OFF AND NOTICED VENT OUTLET VALVE WAS OPEN. THE VALVE THEN RETURNED TO NORMAL CLOSED POSITION AND AIRCRAFT WAS ABLE TO PRESSURIZE AGAIN. REMOVED AND REPLACED AEVC PER 21-26-34. OPS CHECK NORMAL.