OK...6 months ago my check engine light came on in my 1993 300E. After checking things out, and con\sidering it had just hit 80,000 miles, I replaced the oxygen sensor and reset the light. It stayed off fo 2 months and then came back on. Mileage has stayed at a constant 26-28mpg and it runs great. Today my wife mentioned that the car had a heavy duty rotten egg odor under hard pulls like accelerating up hills. Catalytic convertor? Any thoughts?