The O2 sensor should not be replaced unless it's causing problems. If Cal vehicle, you would a check engine light, however the best way is to have some one check the on/off ratio, basically make sure it's in closed loop. If it is, don't even worry about the thing.
Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture