The Genuine Volkswagen Audi Cylinder Head Non-Return Valve is a critical component located internally within the cylinder head. In high-mileage vehicles, it's prone to failure over time. It's imperative to replace this valve if the cylinder head is removed or if failure is detected. This valve plays a crucial role in regulating the flow of fluids within the engine, ensuring optimal performance and efficiency. Failure to replace a faulty non-return valve can lead to issues such as oil leaks or improper fluid circulation, potentially causing engine damage.