Genuine OEM Audi Coolant Regulator, part number 03G121113. This essential component, often referred to as a thermostat, precisely controls the flow of engine coolant to ensure the engine reaches and maintains its optimal operating temperature. It opens and closes based on coolant temperature, directing flow through the radiator as needed. Using a genuine OEM coolant regulator ensures accurate temperature calibration and reliable operation, critical for engine efficiency and longevity.