I would ask your mechanic why doesn't he just put a new one then? Probably? Where I come from mechanics don't say "probably", they just do it charge you whatever. At anyrate...I would bet it's not the O2 sensor anyway. As I have a 1990 300TE I can offer this. There are several different versions of O2 sensors on the market, both in brand and otherwise. The big difference is the wiring harness and associated ease of installation. I looked into this and found that prices can vary from $40 to $300, depending upon things. In the final analysis of my particular problem however, it was a coolant sensor and new spark plugs. I was thinking way past the problem.......