Genuine OEM Audi Coolant Reservoir Cap, part number 420121321. This essential component is designed to securely seal the engine's coolant reservoir, preventing coolant loss and maintaining the precise pressure required for optimal cooling system operation. It is typically located on top of the coolant expansion tank. Constructed from durable, heat-resistant plastic, it weighs approximately 0.14 kg. Choosing a genuine OEM coolant cap ensures the correct pressure relief valve calibration and a perfect seal, which is critical for preventing overheating and maintaining the longevity of the cooling system components.