What was found
JUST AFTER STARTING DESCENT FROM 38000 FT, LEFT BLEED AIR TRIP OFF LIGHT ILLUMINATED. RAN QRH PROCEDURE 3 TO 5 MINUTES AFTER LIGHT ILLUMINATED AND PRESSED TRIP RESET BUTTON. LIGHT EXTINGUISHED WITH NO OTHER MALFUNCTIONS NOTED FOR REMAINDER OF FLIGHT. ACFT GROUNDED. ACCOMPLISHED NR 1 ENGINE HEALTH CHECK IAW NG-36WC-00001 THRU STEP 5. REPLACED THE NR 1 ENGINE PRECOOLER CONTROL VALVE IAW AMM 36-12-02. INSTALLATION TEST PASSED. REPLACED NR 1 HIGH STAGE REGULATOR IAW AMM 36-11-07. INSTALLATION TEST PASSED. ACCOMPLISHED NR 1 ENGINE BLEED AIR CROSSOVER OPERATIONAL TEST IAW AMM 36-11-00-710-801. OPS TESTED GOOD.