I can't say what would happen if the O2 would go negative as I have never seen it. I have been working on these types of problems since around 1980 when most domestic car started using them and I have never seen a negative voltage come out of an O2 sensor, but one thing I did learn is "Never say that is impossible, because if you do, you will be proved wrong". Again most of my experience is with domestic and very little on euro cars, but most cars used Bosch sensors, like yours. A domestic car won't go into closed loop mode until the O2 sensor shows activity of some sort. I would think staying at or below 0v would keep closed loop from happening.

If you reconnect the EHA but unhook the O2 will the car run reasonably well?

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84 500 SEL (307,xxx miles)