What was found
AT FL360 RT PACK DUCT PRESS 9 PSI LEFT PACK DUCT PRESS 42PSI. GROUNDED - YES. AIR TURNBACK - DIVERSION TIME 105 MINUTES. PERFORMED TASK CARD NG-36WC-00001. REMOVED AND REPLACED KISS SEAL ON PRECOOLER CONTROL VALVE AS PRECAUTION IAW B737-800 AMM 36-12-02-000-801/400-801 OPS TEST GOOD. NO LEAKS NOTED. REMOVED AND REPLACED #2 ENG (450F) THERMOSTAT REF AMM 36-11-05 OPS/LEAK CK PASS REMOVED AND REPLACED #2 ENG PRECOOLER CONTROL VALVE SENSOR REF AMM 36-12-03 OPS/LEAK CK PASS. COMPLETED #2 ENG BLEED AIR SYS HEALTH CHECK REF T/C NG-36WC-00001. ENG BLEED AIR CROSSOVER OPS TEST GOOD. DUCT PRESSURE IS 42 PSI LEFT & RIGHT.