The correct inspection for Diesel equipped vehicles in counties under the EPA heavy pollution area restriction requires a visual inspection of the DPF and/or catalytic convertors. If a roadside inspection takes place the DPS will look for the DPF/Cat to be in place and if not the inspection station will receive a visit and the owner will get a citation. This has spawned the use of "diesel drag" style spintech straight through mufflers (Flowmaster, Aero, Hushmuff) which strangely enough look almost identical to a DPF/Cat. Hmmmmm???