The cats in these cars are two stage cats and actually have what we call a "light off cat" in the front of it. If you look at the outline of the cat you can see that it has a smaller area up front and a much larger area in the rear. The whole idea of two oxygen sensors is to monitor the catalyst efficiency. It does this by monitoring the flucuations of the front sensor compared to the relatively steady, straight line of the rear. If the rear starts to flucuate like the front then you know that the cat is no longer effective. The front sensor is in the head pipe just upstream of the cat and the rear one is located in the cat right behind the light off cat and just in front of the main cat. Hope this helps.


Mark