ARB currently has extended agreements with various local air districts and ports to enforce its diesel-related regulations. Through these partnerships, air districts can write citations for non-compliant vehicles and equipment subject to various diesel regulations, helping support extensive enforcement of the truck and bus regulation. ARB also recently developed an agreement with the United States Environmental Protection Agency for enforcement of rules for drayage trucks and aftermarket parts.

Although partnering districts typically refer violations to ARB for settlement, the district can also issue a citation immediately without waiting for inspectors from ARB to respond. This is particularly useful when the district receives a complaint from a member of the public who has observed a violation.

The US EPA also works with ARB to enforce its Truck and Bus and Drayage Truck regulations under authority of the federal Clean Air Act. In addition to conducting their own investigations and audits, EPA also assists ARB in joint investigations and audits to help enforce the rules and create a level playing field for all trucks that operate in California. In turn, ARB works closely with EPA to ensure consistent enforcement.

EPA also works closely on cases involving illegal aftermarket parts used by diesel powered vehicle owners to gain horsepower. These parts significantly increase emissions and can lead to engine damage and manufacturers’ warranties being voided.

While local and federal enforcement partnerships help leverage ARB’s compliance efforts, ARB’s enforcement teams continue to keep the pressure on throughout the state. This involves a variety of enforcement efforts including multi-agency strike forces, roadside inspections with California Highway Patrol, joint inspections with authorities at selected ports, and random inspections at truck stops and rest areas.

ARB conducts investigations and fleet audits based on tips from the public, from air districts, and from compliant fleets reporting non-compliant competitors to ARB’s Complaint Hotline.

Citations issued by ARB can result in substantial penalties. Monies collected go to the State Treasurer for deposit into the California Air Pollution Control Fund which provides funding for projects and research to improve California's air quality.

In some cases, companies can also place up to 25% of the total penalties into a Supplemental Environmental Project. This could fund, for example, the clean-up of school buses through adding diesel particulate filter retrofits, new engines or new buses. Monies used for school bus retrofits are administered by the California Air Pollution Control Officers Association and are distributed statewide with priority given to small districts with the greatest financial need.