Genuine OEM Audi Coolant Expansion Tank, part number 3D0121407J. This essential component holds excess coolant and allows for expansion and contraction within the engine cooling system, ensuring proper fluid levels and pressure regulation. It is typically located in the engine bay, connected to the radiator and engine. Weighing 1.31 kg and measuring 0.3m x 0.29m x 0.175m, this tank is designed for precise fitment. Choosing a genuine OEM expansion tank ensures correct material composition and pressure resistance, preventing leaks and maintaining the integrity of the cooling system.