[Walt Fricke]

$0.02 undervalues the post. At least 0.03! Perhaps the fact that the dithering also keeps the A/F average ratio where it is wanted for fuel efficiency is a happy convergence of two requirements? Your explanation of the pollution effect is really clear (even if borrowed) and I may even be able to remember it.

The point here is the dithering points to the fact that the narrow band sensor isn't useful as a data source for tuning.

Just for grins, does anyone know what kind of O2 sensor the later, water cooled, 911s have? Wide or narrow? And what general system it plays a role in? Does it, too, dither things to make the cat work properly for a reasonable number of miles?

Aftermarket EFI systems often can use data from a wide band O2 sensor to adjust the fuel map. Of course, with these systems I suspect few care about emissions, unless occasionally for when they have to get tested by the state if street driven.