Actually, there is no such thing as an air-to-fuel sensor! People are fooled into thinking that the OXYGEN sensor can measure air and fuel??
The oxygen sensor's purpose is to detect oxygen- a little or a lot, and gives a corresponding signal of some form- volts or current.
To know the air-to-fuel ratio, you need to know the amount of fuel, and the amount of air, obviously. Electronic fuel injection can do that, using the intake sensors- MAF or AFM, air temperature, etc., then by measuring the pressure in the fuel lines it can "calculate" how much fuel to inject. The oxygen sensor's job is to give feedback to the computer/controller, and if it detects lots of oxygen, it means there probably wasn't enough fuel injected, and vice-versa.
That only works assuming you have an engine that is working perfectly for the E.F.I to get things right for a 100% complete burn. If not, the controller thinks it's running rich or lean and tries to adjust, or eventually gives "errors" or the "check engine" light is illuminated.
Therefore... the oxygen sensor is NOT an air-to-fuel ratio sensor. There is no such thing. It takes a number of sensors and some electronic calculating to get the the air-fuel mix to what it should be, and on the newer E.F.I. systems, you can fool with that too.
The best people (with L-Jet E.F.I.) can do is to make a lot of assumptions, like the spark is good, the spark timing of right for the revs, and then adjust the AFM (tighten or loosen the spring) to get the ECU to add more or less fuel to the cylinders. People who incorrectly think the oxygen sensor can determine the air-fuel directly are making a LOT of assumptions and are not achieving the goal of a correct fuel mixture.
So using ANY oxygen sensor in the exhaust stream to predict the air-to-fuel mixture going into the cylinders is like measuring the fuel level in the fuel tank by putting the oxygen sensor in the filler neck.