Modifying your car is legal, though there are elements of car modifications that are illegal depending on which state you live in. Cold air intakes, for example, are illegal in Arizona, California, New York, Pennsylvania, and several other states if they don't carry a CARB Executive Order (EO) number.

What states is it illegal to modify your car?

The EPA has deemed the practice illegal and states such as Illinois, New Jersey, Colorado and Maryland have passed their own laws banning the modification itself from being made and imposing fines on anyone who is caught on the roads belching black diesel smoke from their trucks.

Why is modifying cars illegal?

Engine modifications

Since California is susceptible to poor air quality and smog, the California Air Resources Board (CARB) studies aftermarket parts before approving those that will be permitted for installation on an on-highway driven vehicle or car.