As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRUISE FL 380 LEFT PACK DUCT PRESS 6 PSI RIGHT IS 40 PSI ON CLIMB OUT RIGHT 42 PSI LEFT 5 PSI. GROUNDED-YES. C/W NR 1 ENGINE BLEED AIR HEALTH CHECK T/C NG-36WC-00001, FOUND PRECOOLER CONTROL VALVE FAILED CONTROL PRESS CHECK. REMOVED & REPLACED NR 1 ENGINE PCV, PER B737-800 AMM 36-12-02. PCV OPERATIONS CHECK GOOD. COMPLETED ENGINE BLEED HEALTH CHECK T/C WITH NO FURTHER DEFECTS NOTED. PERFORMED ENGINE BLEED AIR CROSSOVER OPERATIONS TEST, PER AMM 36-11-00. OPERATIONS TEST PASSED. GROUND VERIFICATION TEST SUCCESSFUL. LEFT & RIGHT DUCT PRESSURES W/IN LIMITS.