Last summer, I installed an
ancient old PLX M-250, and it takes the place of the pre-cat O2 sensor. It has a wideband output suitable for an aftermarket engine computer or a gauge, and a narrowband output suitable for the stock engine computer. It enables me to determine when my truck goes out of closed loop operation when I accelerate, and is pretty neat to watch.
I could build a circuit that could take the wideband output, and generate a narrowband output in the desired AFR range, and that would only take effect at 20% throttle opening or less. Above 20%, the circuit would revert back to an unmodified signal. I'd still have to replace the post-cat O2 sensor with my other M-250 that I have sitting around, and have this circuit modify that signal as well, so as to successfully fool the stock engine computer.
However, that is something that we would do. I imagine that most people who buy these EFIE things do not actually take the effort to ensure an accurate AFR by buying a $300 or so wideband O2 sensor to monitor their output. What do they do, instead?