i understand the Oxy sensor is a thermo couple, that is it produces voltage when reach working temp.
i read from wikipedia on these gizmos.

200 mv = engine running lean so ECU will dump more gas in.

450 mv is ideal as normal function.

800mv = over rich so ECU cuts fuel.

ANother trick if needed to pass Emission tests, one can parallel a small 1.5v battery and a rheostat ( a small volume control ) to adjust the voltage. U have to make sure the polarity or else can be very costly.

There are 3 wires, a common ground and a positive power for the heater to the O2 sensor.

I was told Merc run the wire thru the right side of trans tunnel to the pass side foot well. If u pull the carpet, a piece of plastic board almost 60 degrees for foot rest, use a 10 mm socket to remove the panel. The ECU should be below it. See if the wires feed thru.

Imight try to find a used O2 sensor, there's one in my area wanted 40 and had passed emission few mths ago.

let us know what happened.
cheers