Classic cars do qualify for a smog exemption in California. By definition, a classic or antique car is any vehicle that is 25 years or more old. Since cars built before 1975 are exempt from the California smog certification program, owners are not required to have their vehicles checked for smog-producing emissions. This include oxides of nitrogen, hydrocarbon emissions and carbon monoxide emissions. Pre-1975 cars have rudimentary emission control systems that usually consist of exhaust gas recirculation valves, as well as early manifold air pressure sensors. However, this is about the limit of any emissions add-on hardware. The other cure used was leaning out the fuel-air mixture as much as possible.