A timing belt is a flat rubber belt reinforced with fiberglass or similar strong fibers with rubber teeth molded onto the surface. These teeth are driven by teeth on the camshaft and crankshaft sprockets. Exposure to extreme stress and temperatures does require periodic replacement of the rubber belt. Engine oil or coolant could contaminate the belt requiring premature replacement.
Most modern VW and Audi engines are of interference (also called non-freewheeling) design. This means the pistons and valves are close to each other so if the timing belt breaks they will crash into each other and cause serious engine damage. Replacing bent valves can be quite costly.
Many timing belt failures result because of failure in one of the other components of the timing belt system. The other components – an idler or roller pulley, a timing belt tensioner, and a water pump are driven by the timing belt and have internal bearings that may fail. For example, if the water pump seizes or comes apart, it would cause the belt to overheat and self-destruct. It is important not to ignore squealing or unusual noises coming from the engine area as this may be a warning that one of these other parts is about to fail. Immediate action may help to avoid major damage and more costly engine repairs.
It is highly recommended all components of the timing belt system be replaced at the recommended service intervals.