Poor or faulty air mass sensors on the 1.8T's can cause numerous problems without setting a check engine light. Typically owners do not notice the decrease in performance driveability or fuel economy till the problem gets really noticeable. Air mass sensor usually die a slow death so most customers notice a huge improvement in performance smoothness & fuel economy when replacing their faulty air mass sensor with a new one. It is not uncommon to have to replace a air mass sensor every year especially on cars that have been modified.
Replacing air mass sensors is one know solution for "lean" or "rich" engine codes

VIN split Applies to A4 97-01: Select correct model with the drop-down menu below
AEB engine code, up to VIN #W-175,000 split is in the 1998 model year - OEM Part Number: 0280217112
AEB engine code, from VIN #W 080 001, split is in model year 1998 - OEM Part Number: 037906461C (Bosch # 0 280 217 117)