Sugar Bear, Thanks for your post. I was under the impression there are two 02 sensors, upstream and downstream. The 02 sensor is only called upstream, even though the 02 sensor is located on the exhaust bottom, basically downstream. That explains why I was never able to get a downstream 02 sensor for my particular 1994 mercedes E320 wagon in auto parts stores because it does not exist. Therefore, there is only one and only one 02 sensor overall on my 1994 Mercedes E320 wagon. Boy, am I getting an education here. I felt stupid!!! Oh well, live and learn.