19300-ZX2-003 Thermostat Assembly - Overview. The 19300-ZX2-003 Thermostat Assembly is a vital component within Honda marine engine cooling systems. Its primary role is to regulate engine temperature by controlling the flow of coolant, ensuring the engine operates within the optimal temperature range. Proper temperature regulation is critical for engine efficiency, performance, and longevity, making this thermostat assembly indispensable for Honda marine engines.. Key Features. Regulates engine