Genuine OEM Audi expansion tank, part number 8W2121405N. This component serves as the reservoir for engine coolant, allowing for expansion and contraction due to temperature changes. It plays a vital role in maintaining the correct coolant level and pressure within the cooling system, preventing overheating. Constructed from robust plastic, it is designed to withstand the thermal stresses of the engine bay. Genuine OEM parts ensure the correct material composition and structural integrity for reliable cooling system performance.