Yamaha - Thermostat Engine Cooling 66M-12411-01-00. Overview. Keeping the engine at the right temperature is critical for small and mid-range Yamaha outboards. The Yamaha 66M-12411-01-00 Thermostat regulates coolant flow through the engine so it warms up properly and maintains stable operating temperatures while running. When a thermostat sticks open or closed, the motor can run too cold, lose efficiency, or start creeping toward overheating. Replacing the thermostat during routine cooling syste