I have a '04 GT with the factory mid-pipe, which means I have two "pre-CAT's" and two regular CAT's. Do I need to replace all four with these high-flows, keep the two "pre-CAT's" and replace the two downstream CAT's with high-flows, or replace the two "pre-CAT's" with pipe and replace the two downstream CAT's with these high-flows. I am only concerned with keeping the Service Engine Soon light off.
These are designed to replace the 2 cats closest to the manifolds. You do not need to run 4, you can run 2. Since these are high flow, you will still need to run SVE-MILO2 on your rear O2 sensors to keep the check engine light off.