Drain out about 1gallon of antifreeze then take off the top hose the thermostat in underneath it and it sort of looks like the top of a military tank or an observatory with a telescope sticking out on 1 side it might be plastic or aluminum loosen the bolts and remove the thermostat housing and gasket clean the suface with cleaner then remove the thermostat itself and replace the old with the new gasket and thermostat make sure all surfaces are clean and wire brush the bolts ONLY so they tighten easily and till snug then 1/8 of a turn more fill with antifreeze and check for leaks then drive the vehicle and inspect for leaks there should be none after the engine cools down and still no leaks your good if you see any 1/8 of a turn again but no more if your gasket is a rubber ring no sealant is required if it's a hard gasket sealer is required and after tightening must dry for 1 hour before adding antifreeze I don't see a listing for a 2003 in the U.S. SO I CAN'T BE SURE WHAT I TOLD YOU IS RIGHT BUT I SEE A 2006 WITH A 2.4L GO TO ROCKAUTO. COM AND SEARCH FUSION YEARS THEN YOUR PART OK