Since non foulers pull the O2 sensor away and out of the direct exhaust stream, wouldn't that cause the car to run somewhat poorly b/c it's not reading accurately?

 

from evomt.net

The idea behind it is when you remove the cat the 2nd o2 sensor is going to have too much crap reaching the sensor that isnt filtered out, which will make it think the cat has gone bad. What this does is it moves the o2 sensor further away so a limited amount of exhaust gets to the o2 sensor, it is a VERY little ammount that actually gets to the sensor but as long as it is considerably less than the primary o2 sensor then it wont cause a CEL.

 

I understand that, but the CEL doesn't concern me. Is the secondary O2 sensor in the cat responsible for adjustments in air/fuel/timing? Or is that the job of the primary O2 in the header?

 

my understanding is that the O2 sensors in the cats determine your cat's efficiency and used for emissions only, but don't quote me on that