Motor vehicle manufacturers have been required to meet increasingly stringent new vehicle air pollution standards. But vehicles that are poorly maintained or that have malfunctioning emission controls often emit pollution at levels exceeding these standards. Even seemingly minor malfunctions can cause increased emissions. Major malfunctions can cause emissions to skyrocket by hundreds of times beyond vehicle standards. Even though cars having malfunctioning emissions controls comprise a small portion of the fleet, they can have dramatically higher emissions. Drivers typically don't think of their own vehicle as a significant source of air pollution, but cars and trucks collectively are the single greatest contributor in this country to carbon monoxide pollution and to ground-level ozone, or smog. In our more polluted cities, vehicles contribute between 35 and 70 percent of ozone-forming emissions and 90 percent or more of carbon monoxide emissions. Ambient concentrations of one or both of these pollutants exceed national air quality standards in virtually every major urban area of the country. Substantial reductions in vehicle emissions are essential in our cities' struggle to achieve clean air. PUC check programs can significantly reduce vehicle-related ozone-forming and carbon monoxide emissions. A comprehensive PUC check program can also yield substantial reductions in nitrogen oxide emissions.