As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PIT - FLT 670 - UNABLE TO PRESSURIZE AIRCRAFT AFTER TAKEOFF. RIGHT PACK ON MEL AND LEFT PACK VERY WEAK. FLIGHT RETURNED TO PIT WHERE LANDING WAS WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED RIGHT TURBO FAN VALVE AND FLOW CONTROL VALVE. REFERENCE 95ZZZW421.