Too Lean Fault Codes

[ok2000A8]

I've got a 2000 A8 with 2 fault codes: "17546 & 17544. Fuel Trim Banks 1 & 2. System Too Lean." Anyone know what problem might be? Car is driving well and I wouldn't know of problem if it wasn't for fault code and Engine Light on cluster. Doubt it's fuel pump as car is accelerating and powering well. What do you think? Vacuum leak, if so where? Fuel filter or fuel pre-filter? On code 17544, it also indicates "intermittent". Don't you just love those "intermittent" problems? Thanks for any advice before I start what could be expensive and lengthy exploratory surgery. Owen

[ok2000A8]

Update. Finally solved problem. After some vacuum hose changes that did nothing and Pfaff Tuning who could only refer me to their full Audi dealership, Pfaff (which would be okay I'm sure) I took car back to my trusted (but often expensive) dealer, Agincourt Autohaus. Their Audi specialist spent 90 minutes testing and analyzing the car and finally found problem and fixed it! It was a faulty PCV valve. Not a completely broken one but one that was not fully functional at 100% hence the intermittent fault codes. It's been a week now and no fault codes and car runs like new. Now if I could only get my new Nav system to give me traffic reports and my new rims to run true, I'd be really happy