Engine Manufacturers Association v. South Coast Air Quality Management District 541 U.S. 246 (2004) Los Angeles was worried about air quality, so they instituted a requirement that private owners of large vehicle fleets had to purchase "clean fuel" vehicles. The Federal Clean Air Act §209 preempts States and localities from introducing their own air quality standards. However, California is exempt and can develop their own standards.  The only requirement is that California applies for a waiver whenever they institute a standard that is different from the Federal standard. Los Angeles did not apply for a waiver because they felt that the new rule was a purchasing requirement and not an emissions standard and was therefore not covered by the Clean Air Act. The Engine Manufacturers' Association (EMA) sued, saying that Los Angeles' rule was preempted by the Clean Air Act §209. The Trial Court found that the new regulation was legal.  EMA appealed. The Trial Court found that "where a State regulation does not compel manufacturers to meet a new emissions limit, but rather affects the purchase of vehicles, as the Fleet Rules do, that regulation is not a standard." The Appellate Court affirmed.  EMA appealed. The US Supreme Court reversed and threw out the new regulation. The US Supreme Court looked to the language in §209 and found that in the past, the word "standard" had been interpreted by the Trial Court to include only regulations that compel manufacturers to meet specified emissions limits. The Court found that the word "standard" should be defined more broadly to include commands made to purchasers to buy vehicles based on emissions characteristics. The case went back to the Trial Court, who decided that the part of the regulation that was directed towards vehicles owned and operated by the City of Los Angeles was not covered by §209 and was therefore legal. The basic reason that there is a preemption doctrine in the Clean Air Act is that if there wasn't, each State could have their own draconian emissions standards and car manufacturers would have to make 50 different vehicles, which wouldn't be efficient. However, that worry isn't applicable here, so what's the problem?