Genuine OEM Volkswagen Additional Expansion Tank, part number 3D2122407A. This component serves as an overflow reservoir for the engine's cooling system, storing excess coolant during operation and returning it when the engine cools. This helps maintain the correct coolant level and prevents system pressure buildup. Constructed from durable plastic, it is designed to withstand engine bay temperatures and coolant exposure. Ensuring the integrity of the cooling system is vital for preventing overheating and maintaining engine performance.