Yamaha - Thermostat Engine Cooling 6E5-12411-30-00. Overview. A thermostat might be small, but it plays a big role in how a Yamaha outboard runs. The Yamaha 6E5-12411-30-00 Thermostat regulates engine temperature so the motor warms up properly and stays within its optimal operating range. When thermostats stick open or closed, engines can run too cool or start creeping toward overheating. Swapping in a fresh OEM thermostat during routine service keeps cooling performance consistent and helps you