But let me take one more crack at getting you to understand how a narrowband O2 sensor works, and why adding a potentiometer isn't really the 'fine tuning' that you might think it is.
Narrowband sensors are called that because they really read the mixture over a very tiny range. Right around stochiometric. Just a richer than stochiometric, and the sensor 'flops' to a 'full rich' reading. Just a little bit leaner, and it flops to a 'full lean' reading. Here's a diagram of how the sensor's voltage reacts to different mixtures:
This is acceptable no an OEM ECU because all they need to do is find stochiometric, they're not trying to do rich mixtures under boost, or run lean mixtures under cruise. They just want to hit stochiometric to make the cat happy. They do that by having the ECu cycle back and forth over that switch point. If the sensor is reading 'full rich' then it leans the mixture out, slowly, until the sensor flops to 'full lean', then it slowly richens the mixture up again. This is the 'closed loop
operation, the ECU is cycling back and forth over that stochiometric point.
This means, however, that the stochiometric mixture is BUILT INTO the sensor. The narrowband can't be used to do anything other than find the 'switchpoint' at a stochiometric mixture. You can't trick it into finding a different switch point by adding resistance to the sensor signal. Because it will still flop' at the same mixture. Your trick would work on a wideband sensor, because it sends out a varying voltage over its whole range of mixtures it senses. Adding a resistor would make a difference on a curve like that: