Yamaha - Thermostat Engine Cooling 6H3-12411-11-00. Overview. A thermostat is one of those small parts that does a big job every time the motor runs. The Yamaha 6H3-12411-11-00 Thermostat helps regulate engine temperature by controlling coolant flow through the cooling system, so the motor can warm up properly and stay in its operating range. When a thermostat starts sticking, you can end up with an engine that runs too cool, too hot, or just never feels right. Replacing it during cooling system service is a smart move that helps keep older Yamaha outboards dependable on the water.. Key Features. • Temperature Control - Regulates coolant flow so the engine can reach and maintain proper operating temperature.. • Supports Cooling System Performance - Helps prevent overheating and uneven temperature swings during operation.. • OEM Yamaha Fit - Built to Yamaha specifications for proper installation and dependable function.. • Smart Preventative Maintenance Part - A good replacement item du