This service manual provides comprehensive diagnostic and troubleshooting information for Mercruiser ECM 555 marine engines. It covers a range of sterndrive and inboard models, including the 4.3L MPI, 5.0L MPI, 350 MAG MPI, MX 6.2 MPI, and their Horizon variants, with serial numbers starting from 0M300000 for sterndrives and 0M310000 for inboards. These engines are engineered for reliable performance, featuring Multi-Port Fuel Injection (MPI) and advanced Engine Control Module (ECM) technology to optimize power and efficiency across various displacements. The purpose of this manual is to equip owners, technicians, and mechanics with the knowledge necessary to maintain and repair these Mercruiser engines. It details general information, in-depth troubleshooting procedures for ECM 555 related issues, and includes essential wiring diagrams. Coverage extends to key systems such as fuel, ignition, sensors, and ECM operational parameters, addressing common problems like engine performance issues, starting difficulties, fault codes, and sensor malfunctions. Diagnostic tools and methods are explained to facilitate accurate problem identification and resolution.