Is there any way to test the o2 sensor in my 1989 190e 2.6?

Yes. An O2 sensor is like a chemical battery that produce a current which varies with the concentration of O2 reacting with the metal inside the tip. They ALL work the same way and ALL produce the same current/voltage in a certain concentration of O2. I forget the numbers but if you do a search on this or go to the library you'll find the range (which I believe is 0.0 - 1.0 mV) and the number at a a fual:air ratio of 1:14.7 (0.7 sounds close but, don't quote me).

So, leave the O2 sensor in the exhaust pipe. Find the leads from the sensor and probe them with a meter. Start the engine and watch the meter values change. They should always fluctuate and eventually settle around the ideal number.

That checks the sensor directly. In your diagnostic port you can check the ratio or dwell of ON/OFF time of the O2 sensor. Same info in a different format AND it has been processed through a computer.

Good luck,
Intruder