Genuine OEM Volkswagen Thermostat, part number 5C0121113. This thermostat is essential for maintaining the engine's optimal operating temperature by controlling the flow of coolant to the radiator. It is located at position 4 on the provided diagram. The thermostat's precise calibration ensures efficient engine performance and fuel economy. Genuine OEM thermostats are manufactured to exact specifications for reliable operation and a long service life, preventing engine damage due to overheating or undercooling.