What code did you get?

I think that on the V8, you cannot get to the sensors from the top. Follow the manifold out of the engine, there is a coupling where the manifold bolts to the CAT. The sensors you are looking for are an inch or so after this couple. There is too much stuff in the way on the V8 to see the sensors from the top. I have the 3.0L, and the sensors can be changed from the top.