Genuine OEM Audi Coolant Expansion Tank, part number 4G0121403AG. This vital component serves as a reservoir for excess engine coolant, accommodating volume changes due to temperature fluctuations and maintaining optimal system pressure. It is integrated into the engine cooling system. Made from durable translucent plastic, it weighs approximately 0.786 kg. A genuine OEM expansion tank ensures proper fluid levels and pressure regulation, which is essential for preventing engine overheating and prolonging the life of cooling system components.