Originally Posted by madmax

Think you are exempt and can do what you want? Think again. From a SEMA newsletter (and I believe all this to be technically accurate)

Memo 1A.
This refers to a U.S. Environmental Protection Agency (EPA) memorandum that says it's illegal to remove, disconnect or disable a required emissions control device on any pollution controlled motor vehicle (PCMV), pretty much any 1968 (1966 in California) or later model year car/light duty vehicle used on public roads. What this means is that even if you don't have a local emissions-testing requirement, technically you can still break federal law by chucking/disabling your vehicle's smog equipment.

Federal Emissions Requirements.
If your area has emissions testing, almost certainly some form of underhood inspection goes along with it. Technicians will check to make sure that all required smog equipment is installed and functioning properly. This does not mean you can't change things! You just have to make sure changes you make are smog legal and comply with Memo 1A. Because you may need to prove compliance to an inspector, most product manufacturers offer documentation that states their products are at least "49-state smog legal," effectively claiming that their products satisfy the EPA requirements. One warning: Products that claim 49-state smog legality are not legal for use in California (or states that use California emissions standards) on PCMVs.

California (CARB) Emissions Requirements.
If you live in California or states that use California emissions standards, then Memo 1A/49-state smog legal isn't enough. Your product must have an Executive Order (EO) number issued by the California Air Resources Board to be legal for use on a PCMV, unless it can be considered a replacement part. That means the manufacturer of the product must obtain an EO number before the product can be advertised or sold in California. The manufacturer must also provide an underhood label with the part that inspectors can use to verify that it's appropriate for the vehicle it's on. Products that have EOs are often called "50-state smog legal" because having an EO satisfies both California and EPA requirements. This is pretty cut and dried except for some products like intake manifolds, which can qualify as replacement parts even though they're made by aftermarket companies. In these cases, many aftermarket companies include documentation explaining the status of these parts.