Well, it seems as though the diesel world is starting to gain on gas engines in terms of technology. Apparently most big players are now using a catalyst system to control NOx by injecting urea fluid into the exhaust upstream of a special catalytic converter. The fluid reacts with NOx to break it down to it's original components, nitrogen and oxygen. In order to monitor catalyst effectiveness and to aid in particulate filter regeneration, some manufacturers are using a dual purpose sensor which monitors O2 and NOx levels. While the article I read didn't say the O2 portion of the sensor was a wide band type, it did say the sensor was used to monitor O2 levels for particulate filter regeneration downstream of the converter which suggests that it needs to provide more information than just "stoich" or "not stoich." For those interested in getting a better handle on exhaust emissions while tuning it might be worthwhile to investigate this a bit deeper.