What was found
AT CRUISE FL340, LEFT "BLEED TRIP OFF" AMBER LIGHT ILLUMINATED, LEFT DUCT PRESSURE INDICATED ZERO PSI. WE PUSHED TRIP RESET BUTTON PER QRH PROCEDURES AND BLEED TRIP OFF" LIGHT EXTINGUISHED AND LEFT DUCT PRESS INDICATION MATCHED RIGHT DUCT PRESS. IND. GROUNDED - YES. ACCOMPLISHED #1 ENG BLEED AIR SYSTEM HEALTH CHECK TASK CARD NG-36WC-00001 & FOUND PRECOOLER VALVE FAILED TEST. REMOVED #1 ENG PRECOOLER VALVE REF B737-900 AMM 36-12-02-000-201 & INSTALLED SERVICEABLE PRECOOLER VALVE REF AMM 36-12-02-400-801. LEAK & OPERATIONAL TEST GOOD. REF AMM 36-11-00-710-801 & TASK CARD NG-36WC-00001 FOR ENGINE HIGH PWR RUN & HEALTH CHECK TASK CARD NG-36WC-00001. DUCT PRESSURE WITHIN LIMITS NOW.