first off i have 03 Altima 2.5S, getting dtc code p0138(O2 sensor (bank 1 sensor 2)), I was told that the O2 sensor (bank 1 sensor 2) was after the cat on this car but there is nothing but pipe from small cat back, now that is said, i do see 2 diff O2 sensors coming off the manifold b4 the cat. is one of them the O2 sensor (bank 1 sensor 2). if any one knows where to get a diagram for this on the internet then please send me the link.