Just tested again to make sure. With engine running and wire inserted into plug, I get a reference voltage of .49 dc. When I ran operational driving tests months ago, the results fell in the range of .25 vdc and .98 vdc. Per the MB Service manual, I remember that I simulated a "lean" condition by grounding the signal wire leading to the ECU. The O2 sensor then performed like it should by reading the max .98 vdc. Based on the above, I concluded that the O2 sensor is performing as designed.

OVP relay is probably original (I purchased the car at 37k mi and I am now at 120k). Can I test if voltage to pin 1 is functioning with the ECU unplugged?

Why do you mention the coolant temp sensor? Would this factor into the fault code?