California cars operating on higher-sulfur gasolines than prevalent in California should not result in “false tripping” of the on-board catalyst monitoring system. The monitoring systems to be used on California low-emission vehicles will be unable to detect the small extra amount of loss in catalyst effectiveness that might be caused by higher sulfur levels outside of California. Testing of a mocked-up ultra low-emission vehicle (ULEV) conducted by the California Air Resources Board (CARB) confirms that on-board monitoring systems are not tripped even by high sulfur levels (800 ppm). An independent U.S. Environmental Protection Agency (EPA) assessment also agrees that “false tripping” of monitoring systems of the California cars is not expected to be a problem either for California cars operated temporarily outside of California or for California-type cars sold in other states.However, future evolutions of the provisions in the monitoring regulation and in the technology of catalysts and monitoring systems might affect these conclusions somewhat, especially for cars meeting ULEV standards. The regulatory provisions and the hardware capabilities must continue to be matched, as they are at present, to avoid the potential for “false tripping” of the catalyst monitor.In general, the issue of sulfur levels should be regarded more broadly than one of false tripping of on-board monitoring systems. Fuel suppliers, automakers, and regulatory agencies need to work together to balance catalyst costs and fuel costs, and to arrive at approaches that deliver the emissions reductions needed to meet air quality objectives in the most cost efficient manner. The optimization should consider catalyst system costs, performance and costs of on-board monitoring systems and inspection and maintenance (I&M) programs, and costs of gasoline. At present, on-board diagnostic (OBD) tripping does not appear to be the most important factor affecting the balance.