Genuine OEM Audi Coolant Expansion Tank, part number 4E0121403G. This essential component of the engine cooling system is responsible for holding excess coolant and accommodating its expansion and contraction due to temperature changes. It is typically located in the engine bay. Weighing approximately 0.846 kg, this tank is constructed from durable, heat-resistant plastic. Using a genuine OEM expansion tank ensures proper pressure regulation, material compatibility with coolant, and resistance to thermal cycling, which are critical for preventing cooling system failures.