Genuine OEM Volkswagen Expansion Tank Cap, part number 1H0121321C. This cap is designed to seal the engine coolant expansion tank, maintaining the correct pressure within the cooling system and preventing coolant evaporation or leakage. It is a critical component in the engine cooling assembly. The cap is made from durable blue plastic and weighs approximately 0.0591 kg. Using a genuine OEM expansion tank cap ensures the correct pressure relief valve setting and a perfect seal, which is vital for preventing engine overheating and maintaining cooling system integrity.