Exempted parts are add-on or modified parts that have undergone an CARB engineering evaluation. If the part or modification is shown to not increase vehicle emissions, it is granted an exemption to emission control system anti-tampering laws. This exemption is called an Executive Order (EO) and allows the modification to be installed on specific emission controlled vehicles. Every Executive Order part or modification has an assigned number that can be verified by Smog Check stations, BAR Referee stations, or by the CARB.
For certification purposes, vehicles are grouped in engine families or test groups for exhaust emissions (hereinafter, engine families) and evaporative families for evaporative emissions. Within each family, the vehicles share similar designs and are expected to have similar emission characteristics. An Executive Order is issued to certify an engine family in combination with one or more evaporative families. An Executive Order is valid for production during the specified model year. Vehicles produced for another model year require another Executive Order.