Genuine OEM Volkswagen Coolant Expansion Tank, part number 6Q0121407D. This tank is a critical component of the engine cooling system, designed to hold excess engine coolant and allow for thermal expansion and contraction of the fluid. It helps maintain proper system pressure and ensures the engine's operating temperature remains stable. Constructed from durable, translucent plastic, it features clear 'max' and 'min' level markings for easy fluid level checks. Choosing a genuine OEM expansion tank ensures precise fitment, correct pressure relief valve operation, and resistance to thermal cycling and chemical degradation from coolant, preventing leaks and system failures.