So from your explanation these O2 sensors work differently than what I was told by the GM guy. These actually produce their own voltage signal. If I could figure out what those voltage ranges are that the PCM is looking for I'm certain I could build my own bypass box. I'd imagine that if a certain constant voltage is looked at as a bad sensor, that maybe an oscillating voltage within a certain range should trick the PCM into thinking everything is okay? I'm pretty sure cars from the '90s don't use the back sensor for anything besides determining CC health, so I wouldn't have to worry about it affecting anything else.
When I have a problem like this I like to gather as much info as possible and fix it myself rather than buy an off-the-shelf fix someone else designed without understanding how it works. Then I can see afterwards if my design works better/worse than whats on the market, and if mine is cheaper/better/easier I can post the results for others to try. I don't need you to design a box for me, I just need to know what the PCM is looking for from the rear sensor and preferably which wire/wires its looking at. At that point I can create a box that does pretty much anything.
I will start with the burn procedure though. If that fixes it I'll just call it good and wait for some other interesting issue to break out my soldering tools and PC boards :)
I would appreciate that wiring diagram for the O2 circuit. Especially if it references what each of the 4 wires do that go to the O2 sensor and what color wires they should be. The colors I mentioned before (black, grey, white, and white) are from the sensor pigtail, and the car side of the connection uses other colors. I doubt it would help, but I could look at what colors they are tomorrow if it helps to locate them on a diagram.
No big hurry, I'll be tied up with Easter and family activities here for the next few days. Happy Easter!
If burn off procedure doesnt work let me know and I'll see if someone else has some suggestions...Maybe someone else has built one but I havent so I woulndnt be able to tell you how..Let me know, thanks.