Cons-you cannot license your car in an emissons area, cannot sell it there (here in Colorado the seller is financially liable for emission test failure), and if the area you live in becomes an emission test area you are stuck. Test areas are growing, not shrinking as new EPA limits are causing more areas to fail. I would be sure to save any parts you take off as replacing them is expensive (especially catalytic converters). The emission testing program here started suddenly in the 80's and caught most of us by surprise just as we were celebrating the death of the annual safety inspection program.