Genuine OEM Volkswagen Coolant Expansion Tank, part number 1S0121407J. This component stores and pressurizes engine coolant, accommodating expansion and contraction due to temperature changes to maintain optimal system pressure. It is located at position 23 in the diagram, within the engine bay. Typically made of durable plastic, it includes connections for hoses and a pressure cap. Genuine OEM expansion tanks are designed with precise material specifications and sealing surfaces to prevent leaks and ensure long-term reliability.