In article <jGD6h.10538$yl4.5877@newssvr12.news.prodigy.com>, Carl 1 Lucky Texan wrote: The front sensor is exposed to a different, hotter mix of gasses. The rear one is to monitor cat efficiency. I can see their functions being different enough to warrant specific designs. The older 'narrow band' type certainly can survive and do the job to control mixture well enough. Wideband