You will not have a problem installing an aftermarket muffler on your Mazda 3 S. The smog inspection does not inspect the muffler or exhaust system AFTER the CAT. The catalytic converter, on the other hand, is inspected and must conform to your vehicle exact year, make, model and engine size. Be certain the exhaust system you intend on installing has no attachment for any emissions sensors (ie oxygen sensor) and does not require the removal of any catalytic converter (except for during installation).
