Just got my 2004 A8L back from the shop, Timing belt.power steering pump,water pump, transmission fluid and filter, and oxygen sensors. The car has 150,000 miles. Check engine light has been on with codes p0174, p0491, p0492 lean running condition. secondary air too lean right and left banks. After all the work the codes returned in about 100 miles. I thought the new oxygen sensors (both upstream and downstream) might cure the codes. nope. The car runs great excellent fuel mileage (before and after work) starts easily and idles well. Where do I look for the secondary air pump, how can I check it? Should I place a piece of black tape over the check engine light? (just kidding.) Any suggestions will be appreciated. Jetsam

 

Clean your MAF sensor? Also there are some vacuum lines associated with the secondary air pump, make sure you don't have a vacuum leak. I don't remember the location of these, but there were some threads explaining the issue.

 

shot all around the vacuum lines with starting fluid this afternoon and there was no increasing idle speed . Have a new MAF on order should be here tomorrow, My mechanic ordered a "smoke machine" to look for any small vacuum leaks. He said he has wanted one for a while and this is a good excuse to get one. I am always glad to help a mechanic spend his money. Jetsam