Genuine OEM Audi Coolant Expansion Tank, part number 5Q0121407Q. This vital component of the engine cooling system is designed to hold excess coolant, allowing for thermal expansion and contraction as engine temperatures fluctuate. It helps maintain proper coolant levels and system pressure, preventing air pockets and ensuring efficient cooling. This plastic tank weighs approximately 0.178 kg and measures 0.16m x 0.145m x 0.15m. Opting for a genuine OEM expansion tank guarantees the correct material durability, pressure resistance, and precise connection points, crucial for preventing leaks and maintaining cooling system integrity.