Bingo, O2 sensors work off of heat. As a matter of fact that is how you test a sensor to see if it works, heat it with a torch with a Multimeter hooked up. And they are closer to 2 feet farther downstream so the temps could be quite different.

 

Oic, Interesting stuff ...

I never really gave it much thought. I see how that could throw the readings off though. I'll have to borrow my uncles snap on scanner and make a few WOT runs to see what mV readings I get out of my O2's with my headers on. I remember a while back before my headers, at WOT, they would stay in the 700-800mV range. That seemed right to me since they run rich from the facotry under WOT, and they should fluctuate from 100-900mV, the higher the voltage being richer ...

I'll keep this in my email and post a reply whenever I can borrow his scanner and take some readings now with the headers, and mileage, to see how far they vary from the baselines ... This was on a 60 degree day with the car hot and idling for several minutes, so it should be easy to duplicate. I know the humidity will be off, but I would think it should be close enough to show a difference I'll reply whenever I get a chance too though so it ma be a while cause its not too often I can get my hands on a several thousand dollar tool.