The Yamaha YFZ450S, produced between 2003 and 2004, is a high-performance All-Terrain Vehicle (ATV) engineered for aggressive trail riding and off-road racing. Drawing power from a liquid-cooled, 4-stroke, DOHC, 5-valve engine derived from the R1 sportbike, this ATV delivers exceptional performance and agility. Its design emphasizes rider control and stability, making it a formidable competitor in the sport ATV segment. The YFZ450S is recognized for its groundbreaking contribution to the market, offering a potent combination of power and handling that has earned it praise from riders and industry observers alike. This comprehensive workshop repair service manual provides detailed information for maintaining and repairing all models and engine types of the Yamaha YFZ450S. It covers a wide range of systems, including the fuel, cooling, engine lubrication, and electrical systems, as well as the clutch, transmission, brakes, suspension, steering, and frame. The manual includes step-by-step repair procedures, diagnostic information, troubleshooting guides, and critical technical specifications, such as torque values and tolerances. With hundreds of diagrams, drawings, and photos, this English-language manual is an invaluable resource for owners and technicians seeking to perform routine maintenance, complex repairs, and ensure the optimal performance and longevity of their YFZ450S.