Genuine OEM Audi Coolant Expansion Tank, part number 4H0121403K. This essential component in the engine cooling system serves as a reservoir for excess coolant, accommodating thermal expansion and contraction of the fluid. It helps maintain proper system pressure and allows for visual inspection of coolant levels. The tank weighs 0.851 kg and measures approximately 0.35 meters in length. Choosing a genuine OEM expansion tank ensures correct material composition, pressure rating, and hose connections for reliable coolant system operation.