Dionysius,
I saw a problem like this one time (many years ago) that drove me crazy. O2 readings were negative. I have also seen them too high. I got the car from another shop that just gave up on it. It turned out to be a bad ground for the ECM. Now this was a Buick if I remember correctly. GM back in those days got the ground for the ECM from the engine. The ground at the ECM itself was a few tenths of a volt higher than at the engine. The ground for the O2 sensor was at engine ground.
I am just mentioning this as food for thought.

PaulM

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84 500 SEL (307,xxx miles)