It is unlawful to tamper with a vehicle to the point the emission control system is defeated.  In general one is given a limited amount of time to restore the vehicle back to stock and working emission compliant standards; however in extreme situations a court can ordered the vehicle destroyed.  One /CAN however ADD equipment to IMPROVE the emissions of the vehicle so long as it does not alter or defeat the OEM emissions control system; such as for example burn a cleaner approved motor fuel which works with the OEM fueling system.  You can click on the image below to open it in a new window and expand it to full size to easily view and read it.