This Service Repair Factory Manual provides comprehensive diagnostic and repair information for the Mercury MerCruiser PCM 555, covering serial numbers 0M000000 and above. Engineered for reliability, the PCM 555 system is designed to optimize engine performance and provide critical diagnostic feedback. This manual is an essential resource for understanding and maintaining the complex systems within your Mercury MerCruiser engine. The purpose of this manual is to guide owners, technicians, and mechanics through the diagnosis and repair of the PCM 555 Engine Guardian and warning systems. It details essential procedures, including troubleshooting guides, wiring diagrams, service precautions, and specific instructions for wiring harness and connector service. By following the step-by-step instructions and utilizing the provided reference charts, users can effectively maintain and repair their Mercury MerCruiser PCM 555 systems.