This manual covers the Komatsu PC400-5, PC400LC-5, and PC400HD-5 series excavators. These machines are powered by the Komatsu 6D125-1 or S6D125-1 diesel engine, a 12.5-liter, turbocharged, direct-injection unit producing approximately 260 gross horsepower. Engineered for demanding applications, these excavators feature robust construction, hydrostatic drivetrains, and advanced hydraulic systems with variable displacement piston pumps for efficient operation. Their substantial operating weight, ranging from approximately 40,000 to 44,000 kg, combined with impressive digging depths and precise swing speeds, makes them reliable tools for heavy-duty construction and earthmoving tasks. This comprehensive service manual provides essential information for the maintenance, repair, and operation of your Komatsu excavator. It details critical procedures for engine service, including specifications for the Komatsu 6D125-1 and S6D125-1 engines, lubrication requirements for engine oil, hydraulic oil, and coolant, and performance data such as digging depth and swing speed. Technical specifications for the undercarriage, hydraulic system, and transmission are also included, alongside troubleshooting guidance and safety protocols. This manual serves as an indispensable resource for owners, technicians, and mechanics seeking to ensure the optimal performance and longevity of their PC400-5 series equipment.