Genuine OEM Audi Coolant Expansion Tank, part number 4E0121403H. This tank is a key component of the engine cooling system, designed to store excess coolant and accommodate the thermal expansion and contraction of the coolant as engine temperature changes. It helps maintain proper system pressure and prevents air pockets. Weighing 1.116 kg, this tank is made from durable plastic. Choosing a genuine OEM expansion tank ensures the correct material composition for heat and chemical resistance, precise fitment for leak-free operation, and accurate level markings for proper maintenance.