What was found
FOLLOW-UP TO REPORT OF LOW LEFT DUCT PRESS IN FLIGHT, ACCOMPLISH NR 1 ENGINE PNEUMATIC SYSTEM HEALTH CHECK, PER TASK CARD NG-36WC-00002-1. GROUNDED-YES. ACCOMPLISHED T/C NG-36WC-00002-1 NR 1 ENGINE BLEED AIR SYSTEM HEALTH CHECK. NR 1 PRECOOLER CONTROL VALVE FAILS TO OPEN & OVER PRESSURE ON CONTROL SIDE. NR 1 HSR FAILS TO SHUTOFF. REPLACED NR 1 PRECOOLER CONTROL VALVE, REF B737-800 AMM 36-12-02-000-801 & 36-12-02-400-801. REPLACED NR 1 HSR, REF AMM 36-11-07-000-801 & 36-11-07-400-801. OPERATIONS & LEAK CHECK GOOD. NO DEFECTS NOTED ON ANY OTHER SENSE LINES. ACCOMPLISHED ENGINE BLEED AIR CROSSOVER OPERATIONAL TEST, REF AMM 36-11-00-710-801.