Being that you are running lean this basically means not enough fuel to use up the O2.

Are you getting any missing under accelleration ?
Changed your Fuel Filters lately ?
I believe that the Gas Tank cap not sealing properly can cause Lean Running.

Afraid I am a Diesel man myself and have been for some years.

Also I tend to put my car into local MB Shop for all work, though I like to discuss technical matters with them.

Regards

Mike