This service repair manual covers the Mercury Mariner 20 JET, 25, 25 Marathon, and 25 SeaPro outboard engines from the 2001 to 2004 model years, part of the 1st Generation (2001-2007) series. These 2-stroke, water-cooled engines, with a displacement of 430 cc (26.2 cu in), offer 20 HP (JET) or 25 HP (Standard, Marathon, SeaPro) and are designed for reliable marine propulsion. Key specifications include a 2-cylinder configuration, CDI ignition, carburetor fuel system, and a 2.17:1 gear ratio, with options for manual tilt or power trim. This manual provides comprehensive guidance for maintaining and repairing your Mercury Mariner outboard. It details essential procedures, including maintenance schedules, fluid capacities, and technical specifications such as spark plug gap and recommended fuel type. Troubleshooting advice and diagnostic information are included to help identify and resolve issues. By following the instructions within this manual, owners and technicians can ensure optimal performance, longevity, and safe operation of the Mercury Mariner engine.