As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT HIGH PRESSURE ON TAKEOFF ON FLIGHT 3884. COMPLIED WITH QRH BUT COULD NOT RESET. ISOLATION VALVE WOULD NOT OPEN. DECLARED EMERGENCY AND PERFORMED AIR RETURN TO DEPARTURE. PERFORMED ADJUSTMENT TEST OF LEFT PACK IAW AMM 21-51-00. OPS CHECKED NORMAL, VERIFIED CORRECT OPERATION OF ISOLATION VALVE. AIRCRAFT OK FOR SERVICE.