As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO LAX. AIRCRAFT WAS UNABLE TO PRESSURIZE, DUE TO A VENT AVIONICS SYSTEM FAULT AND THE BLOWER VENT OUTLET VALVE STAYED OPEN. THE AIRCRAFT LANDED AT LAX WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE SKIN AIR OUTLET VALVE PER AMM 21-26-53 AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK.