Genuine OEM Audi Expansion Coolant Tank, part number 4F0121403AA. This essential component of the engine cooling system is responsible for holding excess coolant as it expands due to heat, and returning it to the radiator as the engine cools. It helps maintain proper coolant levels and system pressure, crucial for preventing engine overheating. Made from durable, heat-resistant plastic, it features clear MAX/MIN markings for easy level checks. Installing a genuine OEM expansion coolant tank ensures precise fitment, correct material resistance to coolant chemicals and temperature fluctuations, and reliable sealing to prevent leaks and maintain cooling system integrity.