Original equipment (OE) catalytic converters are designed to last the life of properly tuned and maintained vehicles. Many high mileage vehicles require a replacement converter because the original catalyst was damaged, destroyed, or removed, and the cost of a new OE converter on an older vehicle is difficult to justify. In the U.S., a federal aftermarket converter program has been in place since 1986 (California in 1988) and it has resulted in the replacement of over 50 million converters. Both Federal and California programs have required aftermarket converters to meet minimum performance and durability standards.Increasingly tighter emission standards and durability requirements for new light-duty vehicles have resulted in significant technology improvements in three-way automotive catalysts, however these advancements have not always made their way into aftermarket converters. California amended their aftermarket converter program in 2009, doubling the durability requirements and tightening the emission standards to match the original certification limits of the vehicles.To evaluate the difference in emissions performance between the state-of-the art California Air Resources Board (ARB) aftermarket converters and those offered in the federal market, a test program was designed to compare the two technologies across five LEV I certified vehicles. Federal and ARB converters were aged over a RAT-A cycle to represent 25,000 and 50,000 equivalent road miles of aging. Fresh and aged converters were tested over the FTP-75 test cycle. The ARB converters reduced criteria pollutants by an average of 77% NOx, 60% HC and 63% CO below today's Federal aftermarket converters. The data indicates that significant emission benefits could be achieved by revising federal aftermarket regulations to match those required by California.