Engine warning light?

[leilanidude]

If you replace the CAT with a test pipe, your secondary O2 sensor will detect a failure of the CAT and vehicle power and fuel mileage will suffer. Unless of course, you also buy a secondary O2 sensor that is "rigged" to send a false signal... Yes, those really do exist.

edit to add: The code is not a diagnosis. It is a way to help you find the problem that caused the code to be set.