The Komatsu WA420-3LE, WA420-3L, and WA420 series loaders, generation 3, are robust machines designed for demanding operational environments. These loaders are engineered with a focus on reliability and performance, featuring a powerful Komatsu SAA6D125E-2 turbocharged, water-cooled, 4-cycle diesel engine with a displacement of 11.04 L. The advanced transmission with lock-up and hydrostatic power steering ensure efficient operation, while the substantial operating weight of 22,430 kg and a bucket capacity of 3.7 m³ highlight their heavy-duty capabilities. With a maximum travel speed of 38.0 km/h, these loaders are built to handle a wide range of material handling tasks. This Service Repair Workshop Manual provides comprehensive guidance for the maintenance and repair of the Komatsu WA420 series loaders. It covers essential areas including detailed engine specifications, transmission system information, and critical fluid capacities and change intervals for engine oil and hydraulic oil. The manual also outlines specifications for dimensions, braking systems, and cooling systems, alongside safety guidelines. This resource is intended to assist owners, mechanics, and technicians in performing routine maintenance, troubleshooting issues, and ensuring the optimal performance and longevity of their equipment.