It is funny with emission system. It is illegal to tamper with emission systems that are federally required. By federal law you can not remove a Cat from any car or truck used on public roads. BUT the federal government does not enforce it, it allows the States to do this. Most State do emission by people per square mile this is why Philly has tuff emission rules and has since the 80's and places less then 100 miles away have almost none. In some cases we talk about grandfathering in but most of the year stuff you see in emission are bases on the equipment and the technology the inspection station is required to have. In my town my 96 just was a visual inspection the system where they and that the check engine light was not on. My 2000 they plug in and get readings. The difference? By 2000 all domestic cars and truck where fully OBD2 compliant. In 96 some import and trucks had not switch over yet.

When the federal government gets involved is when you profit from removing emission items. I am not saying if you sell a car that your removed a Cat off of you could get in trouble. I am saying if you charge someone money to remove or do a project that results in the emission removal you could see a fine of up to 10k per instance. The only people I have heard about getting nailed for this is TV car shows. Same is true for safety Items. You charge someone and remove the Airbag there is another 10k. ABS another 10k. This is why you normally do not see garages doing swaps. Everything has to work when done because they risk fines and future owners suing them.