As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PIT - FLT 1597 - CREW WAS UNABLE TO PRESSURIZE AIRCRAFT AFTER TAKEOFF FROM PIT. FLIGHT RETURNED TO FIELD AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE RIGHT TURBOFAN AND THE LEFT MIXING CONTROL VALVE ASSY. OPERATIONAL CHECK AND PRESSURIZATION CHECK GOOD ON GROUND. (M)