Genuine OEM Volkswagen Coolant Expansion Tank, part number 3D0122407B. This plastic reservoir is an essential component of the engine cooling system, designed to manage coolant volume changes caused by temperature fluctuations. Weighing approximately 0.573 kg, it helps maintain proper system pressure. Genuine OEM expansion tanks are made from high-quality, heat-resistant plastics to ensure long-term durability and prevent leaks, which are critical for cooling system integrity.