Genuine OEM Audi Engine Coolant Expansion Tank, part number 80A121405AG. This vital component in the engine cooling system stores excess coolant, allowing for thermal expansion and contraction, and helps maintain the correct system pressure. It is typically located in the engine bay, connected to the radiator and engine. Made from durable, translucent grey plastic, this tank measures approximately 0.235m in length. Using a genuine OEM expansion tank ensures proper fitment, material integrity, and reliable pressure regulation for efficient engine cooling.