Genuine OEM Audi Engine Coolant Expansion Tank, part number 8K0121403Q. This essential component of the engine cooling system serves as a reservoir for excess coolant and allows for the thermal expansion and contraction of the coolant as engine temperatures fluctuate. It helps maintain proper system pressure and ensures a consistent coolant level. Crafted from durable, heat-resistant plastic, it measures approximately 0.3m in length. Using a genuine OEM expansion tank ensures precise fitment, correct fluid capacity, and the necessary pressure retention capabilities for optimal engine cooling and longevity.