California Air Resources Board

California has identified emissions from diesel-powered engines as a toxic air contaminant and a precursor to ozone formation. In order to lower these emissions, CARB has adopted regulations to control diesel exhaust. The diesel enforcement program is focused on achieving compliance with diesel regulation requirements across the full range of regulated vehicles and equipment, especially in communities near highways, ports, and other high traffic areas that are disproportionally affected by diesel emissions. Enforcement efforts utilize two broad methods to identify violations: field enforcement and investigations.
More about this program