As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS NOT GROUNDED: AFTER TAKEOFF CABIN WAS NOT PRESSURIZING, SO WE STOPPED CLIMB AT 10,000 FT. AND RETURNED TO JFK, NORMAL LANDING. FORWARD OUTFLOW VALVE HAD FAILED ON THE PREVIOUS DAY AND WAS PLACED ON MEL AND DEACTIVATED. MAINTENANCE FOUND OUTFLOW VALVE OPEN ON FLIGHT RETURN TO JFK. ACCOMPLISHED DDG 21-00-00-040-939. VALVE DEACTIVATED AT 7% AS PER MEL 21-24A.