Over half the pollutants in America's air come from "mobile sources" of air pollution. These mobile sources include cars, trucks, buses, motorcycles, recreational vehicles, scooters, off road construction equipment, marine engines, generators, and small engines and equipment. Mobile source pollutants include smog-forming volatile organic compounds and nitrogen oxides, various toxic air pollutants such as cancer-causing benzene, carbon monoxide, particulate matter or soot, and greenhouse gases. In addition to adverse effects on the environment, these pollutants are responsible for asthma, heart disease and other illnesses.

The Clean Air Act requires new engines and equipment sold or distributed in the United States to be certified to meet EPA-established emissions requirements to protect public health and the environment from air pollution. The Act and its implementing regulations also contain various requirements concerning testing, reporting, recordkeeping, warranty, labeling, tampering, defeat devices, and vehicle and engine maintenance and alterations. EPA enforces the vehicle and engine provisions of Title II of the Clean Air Act and regulations at 40 C. F. R. Parts 85, 86, 88 through 94, 600, and 1033 through 1068.

EPA may seek civil penalties or injunctive relief (including remediation of the violations and projects to offset excess emissions) for violations of the Act and regulations, and may bring cases in federal district court or through an administrative process. Enforcement actions include cases against a variety of parties, including manufacturers, importers, distributors, and consultants.

Violators are subject to civil penalties up to $45,268  per noncompliant vehicle or engine, $4,527 per tampering event or sale of defeat device, and $45,268 per day for reporting and recordkeeping violations. 42 U.S.C. § 7524; 40 C.F.R. § 19.4. The EPA often uses the Mobile Source Civil Penalty Policy to arrive at an appropriate civil penalty for vehicle and engine enforcement settlements.