Genuine OEM Audi expansion tank, part number 8D0121403D. This part functions as the reservoir for engine coolant, allowing for expansion and contraction of the fluid due to temperature changes. It is typically located in the engine bay, connected to the radiator and cooling system hoses. Made of durable plastic, it is designed to withstand engine temperatures and pressures. Genuine OEM parts ensure exact fit and material integrity for reliable cooling system performance.