The last post hit it right on the head. Let me put this in non-tech speak:
The O2 sensors are highly accurate right at stoich, which is approx. 0.45 volts for our OEM sensors. Anything away from there (leaner or richer than stoich 14.7:1) and your accuracy goes down, and is influenced in a greater degree by the temperature of the tip.
What the PCM does is richen when below 0.45 and lean when above 0.45 (using short term fuel trim). In doing this, it keeps the "average" A/F ratio at stoich during closed loop operation. Always bounces back and forth.
You can't say that .86 volts is xxx A/F ratio for all cars, since there are too many variables. (BTW, 0.94 is about 12:1 on my car.) What I have done is recorded my O2's during a dyno run at a known A/F ratio and use that to tune my S-AFC. Best that I can get w/o using a wideband O2 sensor (like on the dyno).
Bottom line, if the number is going lower, you are probably running leaner, but there is a pretty good precision error on these things away from 0.45.
