Genuine OEM Audi Coolant Expansion Reservoir Tank, part number 8E0121403F. This component is designed to manage coolant volume changes caused by temperature fluctuations, storing excess fluid and maintaining proper system pressure. It also serves as a convenient fill point and allows for visual inspection of coolant levels. Weighing 0.641 kg and measuring approximately 0.28m in length, it is constructed from durable, heat-resistant plastic. Using a genuine OEM expansion tank ensures proper pressure regulation, leak-free operation, and compatibility with the cooling system, preventing potential engine damage.