What was found
DURING FINAL APPROACH, ODOR OF SYNTHETIC OIL IN THE COCKPIT. LT TEMP CONTROLLER IN MANUAL WITH LT DUCT TEMP PEAKED. TURNED IT OFF. ODOR WENT AWAY. ON THE GROUND, TURNED LT SUPPLY SWITCH BACK ON, WAS ABLE TO CONTROL TEMP MANUALLY. OPS CHECKED LT MANUAL TEMP CONTROL IAW MM. OPS CHECKED GOOD. NOTED LT AUTO TEMP CONTROL DEFERRED, DI NR 25133. NO OIL ODOR NOTED DURING OPS CHECK. CLEARED DI NR 25133. FOUND LT SYSTEM COOLING AIR DUCT ON AFT SIDE OF WATER SEPERATOR NOT INSTALLED. REINSTALLED LOOSE DUCT AND TIGHTENED CLAMPS. LT AUTO TEMP CONTROL SYSTEM OPS CHECKED GOOD.