You didn't specify what kind of miles were on the vehicle. If it is under 80k, it should be under warranty, and it would then be the dealers concern. I am trying to get the information correct, and service info is not helping. There are two oxygen sensors in the manifold, one on the side accessed from above, and one at the apex of the bend just before the flange. If this was the only two sensors, that would make the catalyst in the manifold the one that is monitored, and therefore the one that is non functional. I don't remember there being an oxygen sensor behind the second catalyst. In looking at service information, it shows the upper sensor as bank one, and then shows a second sensor at the rear of the second catalyst, making both of them monitored. This would make it more of a guess as to which one to replace, but as saturntech had mentioned, the rear catalyst would be the one to go with. Like I say, I know there is two sensors in the manifold, one pre-one post, but am not sure on another sensor aft of rear cat. If you can determine physically the number of oxygen sensors, that may be helpful. If there is three sensors, then service info is incorrect.