As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB-OUT, CREW NOTICED THE CABIN WAS NOT PRESSURIZING (CABIN WAS CLIMBING WITH AIRCRAFT). THE AIRCRAFT THEN RETURNED TO TAKEOFF POINT. INVESTIGATION OF THE AIRCRAFT REVEALED THE INLET COUPLING ON THE PRIMARY HAT EXCHANGER FAILED RESULTING IN NO AIR TO THE CABIN FOR PRESSURIZATION OR AIR CONDITIONING. ACTT: 8,216.5. HOURS. ACTC: 5,365.