The California smog check does not inspect a vehicle's muffler. You can pass the smog check without one having installed. In fact, late model vehicles (2000 & newer) do not receive the tailpipe (exhaust emissions) portion of the smog inspection. Only an OBD-II, visual, and functional inspection is performed.
Read more... No Tailpipe Smog Test for 2000 & Newer Vehicles
