I own a 1998 E320 and the other day while driving, the "Check Engine Electronics" message appeared. I figured that it was probably one of the O2 sensors that tripped it. I took it to the shop to have them confirm my hunch. Sure enough, the number 2 sensor tripped it.

Question - Does anyone know where the number 2 sensor is located, and is it difficult to replace.

Thanks.

 

I am having a similar problem with the heated O2 sensor.
I bought one of those ODBII scanners for $100 at Advance auto parts and it sure enough spelled out in english what the problem was . I got 2 error codes on the O2 sensor.
I reset the alrm but it came back again once the engine computer completed its diagnostics after a few miles were driven.
I guess I will be buying a new O2 sensor soon. I would guess it is located on the main exaust pipe coming from the engine and facing kind of upwards. You will probably need to drive up on a ramp for both side wheels so as to have quick and easy access to it. I will know more when I get the sensor.
MB has a service called startekinfo.com. You can pay for a 24 hour access and printout all you willever want to know about your car service manuals, etc. It is well worth it.

 

The O2 sensor alarm disappeared after I reset it and never came back.
Must have been a temporary bad connection or deposits on the sensor head that went away.