As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AIRCRAFT DID NOT PRESSURIZE ON CLIMB OUT. CABIN ALTITUDE WARNING AT 19,000 FEET. EMERGENCY DECLARED; FLIGHT RETURNED TO SJU LANDING SAFELY. MAINTENANCE REPLACED GROUND CONDITIONED AIR CHECK VALVE PER AMM 21-21-01-000-804-002 AND ACCOMPLISHED SATISFACTORY PRESSURIZATION SYSTEM GROUND TEST PER AMM 21-31-00.