[Formerly Steve Wilkinson]

Whatever it's sniffing is "complete." i.e. if you put a single sensor in a single exhaust pipe, it will tell you what's happening in that cylinder. If you put a single sensor in the exhaust pipe for one entire bank, it will tell you what's happening in the combination of those three cylinders. And if you put one where the total exhaust system, six into one, comes together, it'll tell you what's happening in the combination of every cylinder's exhaust. So for the purpose of running an air/fuel gauge as a (relatively coarse) tuning aid, six sensors and a six-position switch would be the ultimate; a sensor per bank with a two-position switch is next best; and a single sensor would be lest effective. For the purpose of running an ECU, however, a single sensor reading the entire engine is best.

sTephan