As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMB OUT LEFT DUCT PREESSURE WAS 18PSI AND RIGHT WAS 40PSI UNTIL ABOUT FL300. BY LEVEL OFF AT FL380 LEFT WAS 32PSI AND RIGHT WAS AT 40PSI. ON TAXI BOTH READ ABOUT 20PSI. PRESSURIZATION WAS NORMAL THOUGHOUT THE FLT. GROUNDED - YES. REMOVED AND REPLACED #1 ENG PRE-COOLER CONTROL VALVE. REF ASA T/C NG-36WC-00001.