This factory service manual provides comprehensive diagnostic and repair information for MerCruiser ECM 555 systems, specifically designed for MerCruiser boat motors including sterndrives and inboards. It details the ECM 555 diagnostic module and covers both 10-pin and 14-pin harness configurations. The manual is engineered to assist users in maintaining their marine engines in top operating condition through clear, step-by-step instructions and detailed procedures. The scope of this English-language manual encompasses general information, detailed troubleshooting procedures, and essential wiring diagrams. It includes disassembly and assembly instructions, inspection and adjustment guidelines, and is supplemented with helpful photos and illustrations. This resource is intended for dealers and service mechanics, equipping them with the knowledge necessary for effective diagnostics and repair, ensuring the longevity and optimal performance of MerCruiser marine engines.