2001 A4 System Too Lean Bank One

I have a 2001 A4, and it's showing a code of PO171, "system too lean bank 1". Can anyone tell me what this means, and where I can start to troubleshoot the problem?

Thanks!

Chances are you have an intake system air leak (vacuum leak). If you start the car and let it idle you should be able to hear and air rushing sound. Trace the noise and find your leak.