Since 2005 Audi came out with a new engine-fuel system design call TFSI. Audi is the first manufacture in the world to combined direct petrol injection with turbocharging.
In the FSI technology fuel is injected directly to the combustion chamber. The injector located on the admission side in the cylinder head, is served by a high pressure pump driven by the camshaft and a pressure reservoir shared by all the cylinders.
Excessive wear of the intake cam lobe that drives the high pressure fuel pump limits the maximun pump piston lift, causing the fuel rail pressure fluctuations. The wear on the cam lobe also leads to wear in the base of the high pressure fuel pump cam follower causing the check engine light to come on setting fuel system codes.
Audi has improved the hardness of the intake camshaft lobe for the high pressure fuel pump. In many cases the cam had to be replaced in conjunction with the pump and follower.
This is also one of the reason why we do recommended to replaced the oil more often ( every 5000 miles) This particular car only had 57,000 miles.
I had attached a picture of a 2006 Audi A4 Wagon for more information.
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