It would seem to me you would have a "CHECK ENGINE" light and a diagnostic code if an O2 sensor was bad.

My car has been throwing O2 sensor codes for about six weeks, my codes show that the upstream or front left (and at times, right) sensors are bad. These are easy to change and cost around $100 each. Im not sure WHERE the downstream sensors are.

My standard advice to someone is: Fix it yourself or take the car to some local shop known for expertise with Mercedes products. Avoid most Chrysler dealers.