2006 A6 Avant. 2.8q, Engine ATQ. I have two DTC: P1423 and P1411 for Manufacturer controlled aux emissions control. I’m at the end of my capabilities. Have changed all vacuum tubes, O2 sensors and crankcase ventilation tubes. There might be two possible symptoms: I get only a gas milage of 18.5 mpg at the most efficient driving. The second thing is, and I’m not sure exactly what it is and if it might be related, that at mid/low rpm and little load, I hear a distinct knocking/pining sound. It does not sound like detonation, but rather like a mechanical cause. I suspected that I might have a bad hydro-lifter? Could anyone confirm that or have a different idea?
Having that all said, could anyone see a link between the two symptoms and the error codes? When erased, they come back very quickly. I would be grateful for any leads on this. If you are or might know a person that could take care of it in the central MA area, drop me a line!

Thanks,

Mike