As Larry states the O2 sensor code MOST often isn't the sensor itself. The code sets when the activity of the code is not happening. I had one old 300e with such a code just due to lack of power to the heater circuit of the sensor. It wasn't getting hot quick enough and would cool off at idle.

Any mixture problem (or misadjustment) that overwhelms the ability of the system to adapt will leave the sensor pegged. This is the Number one problem!

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician