Genuine OEM Audi Coolant Expansion Tank, part number 8J0121407. This essential component stores excess coolant and provides space for thermal expansion and contraction of the coolant as engine temperatures fluctuate. It helps maintain proper system pressure and prevents air from entering the cooling circuit. Constructed from durable, translucent plastic, it allows for visual inspection of coolant levels. Choosing a genuine OEM expansion tank ensures correct pressure ratings, material compatibility, and leak-free operation for reliable engine cooling.