Originally Posted by alpac

Replacing O2 sensors (post cat or pre cat) is an easy task on the e53 for a mechanic but did they tell you why they wanted change the O2 sensors? particularly the 4 of them! What error code did they read? O2 sensors sometime may need to be replace but only if they are faulty and never did I see having to replace the 4 of them. That does not sound right to me at all.