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Some more info I found about the E46 323ci; whether or not our cars are similar is TBD:
The primary O2 sensors don't have a frequency or duty cycle, and their output oscillates in amplitude from 0.9 volts (rich) to 0.1 volts (lean). The secondary O2 sensors are much more stable. I don't understand this right now, because I would think they would read lean and oscillate with the primary O2 sensors... I guess the cats dampen the oscillation.
According to what I read, the secondary O2 sensors don't necessarily follow the primary O2 sensors. So, if my circuit was to work, it would have to have a voltage supply independent of the primary O2 sensors, or simply reduce the voltage of the actual secondary O2 sensor readings. I don't know if the ECU expects an oscillation, or if a constant ___ volts will do the trick (doubt it, because if it was that easy someone would have done it by now).
O2 sensors PRODUCE the output voltage. They only work when they get up to operating temp. Prior to this, the car is in a rich "safe" open-loop mode. Once the O2 sensors warm up, they produce the voltage... it isn't supplied to them. The only thing supplied is for the heating element. At this point I believe the 4 wires are heater hot, heater ground, O2 output, and O2 ground.
Solution: I could EASILY program a microcontroller to output a signal between two values (0.5-0.7 volts?). The issue is knowing if there's a relationship between the primary and secondary O2 sensor readings, or how the ECU determines if the sensors are actually working. Does there need to be some oscillation? How often? I don't know. If it isn't picky, I could have it randomly oscillate at a slow, smooth rate. The next step will be to get an OBDII tool with data logging capability, and measure/graph each sensor's output simultaneously. Example attached (I claim no rights to the picture).
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