Yamaha - Thermostat Cooling System 6E5-12411-02-00. Overview. On these smaller Yamaha outboards, the thermostat plays a big role in how the engine warms up and runs. When it's working right, the motor reaches temperature and stays there without running too cold or too hot. I've seen engines run rough or even overheat just because the thermostat stuck open or closed. Swapping it out is a simple fix that helps keep everything running steady.. Key Features. Controlled Engine Temperature - Regulates water flow to maintain proper operating temperature.. Reliable Opening Response - Designed to open and close consistently based on engine heat.. Corrosion-Resistant Build - Built to handle constant exposure to water and marine conditions.. Direct OEM Fitment - Matches original specifications for proper installation and function.. Engine & Boat Compatibility. Designed for Yamaha 2-stroke outboard engines using thermostat-controlled cooling systems. Commonly used on small horsepower motors found