I have owned my 1990 300 SE for a little over a year. The car runs beautifully, has 125,000 miles, and the only problem I have had is a code and light on for the O2 Sensor. I put a new Bosch sensor in shortly after getting the car, cleared the code, and it ran perfectly for about 9 months and no light on. Then the CE light came on and the code for O2 Sensor grounded was on. I replaced the sensor again, cleared code, CE light off, great! But the light has come on again, code for O2 is read. I have a pretty good oil leak from rear main seal that gets on the exhaust and will cause some smoke at times off the exhaust pipe. Could this burning oil affect the O2 sensor? Also, what happens if I run the car with the sensor not working? Thanks for any advice!