California Vehicle Code, section 27156 (VC27156) and the Federal Clean Air  Act(CAA) prohibit vehicle modifications which increase motor vehicle emissions. These "anti-tampering" provisions effectively prohibit the sales of after-market parts which are not functionally identical to the parts they are replacing. However, since most properly engineered parts do not increase emissions, the laws allow for exemptions for the installation of parts proven by their manufacturer and the California Air Resources Board (CARB) not to increase emissions. The process by which these exemptions are granted is via an Executive Order (EO). Each EO is assigned a number which is required on all after-market parts.

The EO process begins with a request from the manufacturer to CARB detailing the device to be installed or sold and the vehicles to which it is applicable. CARB will reply with the requirements for consideration of exemption. For some items it may be as simple as providing a sample of the device to CARB for review; for other items CARB may require emissions testing of one or more vehicles equipped with the device at an approved independent laboratory. If testing is required, CARB will provide a list of the vehicles which must be tested by make, model, year and possibly engine family. The tests required of each vehicle will also be listed.

Note: these are not "Smog Check" type tests but rather laboratory tests, identical to the more extensive and accurate tests required of Original Equipment manufacturers to certify new vehicles for sale.