As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INFORMATION ONLY - AT FL370 RIGHT DUCT 43PSI LEFT DUCT 22PSI. THIS OCCURRED 4HRS 30 MIN INTO FLIGHT. GROUNDED - YES. PERFORMED NR 1 ENG BLEED AIR SYSTEM HEALTH CHECK PER T/C NG-36WC-00001. FOUND PRECOOLER CONTROL VALVE TO BE DEFECTIVE. REMOVED AND REPLACED PRECOOLER CONTROL VALVE REF AMM 36-12-02. LEAK CHECK AND OPS CHECK OF NEW PRECOOLER CONTROL VALVE GOOD AT THIS TIME. OPERATIONAL TEST GOOD.