Genuine OEM Audi Engine Coolant Expansion Tank, part number 4K0121405AC. This vital component manages coolant volume changes due to temperature, maintaining system pressure and preventing overheating. It is typically mounted in the engine compartment, connected to the radiator and engine coolant lines. Manufactured from durable, heat-resistant plastic, it ensures reliable performance under demanding engine conditions. Opting for a genuine OEM expansion tank guarantees precise fitment, correct pressure relief valve operation, and material integrity, crucial for the longevity and efficiency of the cooling system.