Yamaha - Thermostat Engine Cooling System Component 6EK-12411-00-00. Overview. If you spend enough time working on Yamaha four-strokes, you learn pretty quickly that the cooling system is everything. This Yamaha thermostat helps regulate engine temperature so the motor warms up properly and stays within its designed operating range. When it opens and closes the way it should, the engine runs smoother, burns fuel cleaner, and avoids unnecessary wear. It's a simple service item, but replacing a ti