Details how emission control defect reporting programs are used to improve compliance with passenger vehicle emission standards. An emission control defect reporting program is used in California to improve compliance with passenger vehicle emission standards. The defect reporting program was adopted by the California Air Resources Board (CARB) through regulation. The regulation requires the manufacturer of the passenger vehicles to report to CARB how many warranty claims (as a percent) for emission control parts it receives within the warranty period for the vehicle. An excessive percentage of reported warranty claims for a specific emission control part suggests there is a defect in the part that is causing or will lead to increased emissions.