This manual covers the Komatsu 6D125-2 Diesel Engine, a robust and reliable power unit manufactured by Komatsu Ltd. Designed for a wide range of heavy-duty applications, this engine is known for its engineering quality and performance. It serves as a critical component in various industrial and construction machinery, including models such as the D53A-17, D65EX-12, WA470-3, and PC400-6, among many others. The engine variants covered include the 6D125-2, S6D125-2, SA6D125-2, and SAA6D125-2, all powered by diesel fuel. The purpose of this English-language Shop Service Manual is to provide comprehensive guidance for the service and repair of the Komatsu 6D125-2 Diesel Engine. It details essential maintenance procedures, troubleshooting steps, and technical specifications necessary for optimal engine performance and longevity. The manual includes information on General Information, Engine Mechanical, Fuel System, Exhaust System, Cooling System, Lubrication System, Electrical System, Intake System, and Preheating System. With step-by-step guides, complete instruction pictures, diagrams, and lists of required tools, this manual is designed to assist both owners and professional technicians in performing repairs effectively and efficiently.