No problem installing a cat back exhaust system. 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. The CAT must be approved for sale and use in California and posses an Executive Order number issued by the California Air Resources Board. A vehicle will not pass the smog check without having the correct catalytic converter installed. The smog technician will inspect the CAT and verify it is designed specifically for your vehicle. Don't take you car in for a smog check without it.