Genuine OEM Audi Coolant Overflow Tank Breather Line, part number 4G0121081AJ. This line serves a crucial role in the cooling system by allowing air and excess pressure to escape from the coolant overflow tank, maintaining proper system pressure and preventing damage. It is a flexible conduit within the cooling circuit. Weighing 0.042 kg and measuring 0.385 m in length, this line is manufactured from durable plastic and rubber. Using a genuine OEM breather line ensures correct routing, material compatibility, and resistance to thermal and chemical degradation, preventing blockages and maintaining system integrity.