Yamaha - Thermostat Cover Cooling System Housing 688-12413-00-1S. Overview. Cooling systems on Yamaha outboards rely on a few key components working together to regulate engine temperature. The Yamaha Thermostat Cover 688-12413-00-1S protects and seals the thermostat housing so the cooling system can operate the way it should. If a cover becomes corroded or damaged over time, it can lead to leaks or improper sealing around the thermostat assembly. Installing a genuine Yamaha replacement helps ke