Genuine OEM Audi Coolant Expansion Tank, part number 4K0121405AB. This vital component holds excess engine coolant, allowing for thermal expansion and contraction within the cooling system. It helps maintain proper system pressure and fluid levels, preventing overheating and damage. Constructed from durable, heat-resistant plastic, this reservoir weighs approximately 1.01 kg. Choosing a genuine OEM expansion tank ensures precise fitment, correct fluid capacity, and material integrity to withstand engine bay temperatures and coolant chemistry over time.