Genuine OEM Audi Coolant Expansion Tank Cap, part number 1J0121321B. This cap is crucial for sealing the coolant expansion tank, maintaining the correct pressure within the cooling system to prevent overheating and fluid loss. It features a distinctive blue color and a coolant symbol for easy identification. Weighing approximately 0.062 kg, it is designed for a secure and precise fit. A genuine OEM cap ensures the correct pressure relief valve setting and material integrity, vital for the safe and efficient operation of the engine cooling system.