To Jack, I think the sensor really does measure the amount of available oxygen (this is also a relation to the relative amount of unburnt fuel). The catalytic converter is designed to use unburnt fuel to start its own combustion reactions to break down anything left. So there always needs to be some fuel getting past the engine to allow this reaction to continue. By adding more O2 in the beginning the engine's reaction will be more complete leaving less oxygen and unburnt fuel for the sensor to read. The computer is going to think "Uh oh, no fuel over here means too much air". To compensate, the easiest and safest thing for the engine to do will be to add more fuel. Also, the computer adapts to it's environment, to a certain extent, to make the engine run more efficiently. If you take a truck from the beach and load it on a trailer and pull it to the rockies it may have a little bit of trouble at first running on the thinner air. I wouldn't be suprised to see a lean code going the other way because the computer will be used to putting so much fuel in for a given amount of mountain air. Take it to the beach where the air is denser and the computer will wonder why there is so much more air. I have read that some people noticed that the generators caused their mileage to go down intially but then over time of at least a few weeks it improved. This could suggest that the computer is learning to use the new ratio of oxygen to fuel. Personally I think there are too many variables to say yay or nay on how well it works. But I do not think that all of the companies selling the kits are honest about fuel savings.
