As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MIA - FLT 1431 - UPON CLIMB-OUT FROM MIA, CREW WAS UNABLE TO PRESSURIZE AIRCRAFT. AFTER SEVERAL ATTEMPTS TO PRESSURIZE AIRCRAFT FAILED, CREW DECIDED TO RETURN TO FIELD AT MIA. FLIGHT LANDED WITHOUT FURTHER INCIDENT, AND NO EMERGENCY BEING DECLARED. MAINTENANCE FOUND THE LEFT AIR CYCLE MACHINE FROZEN (NO AIRFLOW). REMOVED AND REPLACED THE LEFT AIR CYCLE MACHINE PER MM. OPERATIONAL CHECK NORMAL. (X)