The U.S. Supreme Court has ruled that California cannot prohibit fleets from purchasing diesel engines if those engines meet federal guidelines. But despite that ruling, the agency involved says it will still try to regulate the fleets.

The decision did not involve long-haul semi fleets; however, the district does regulate tractor-trailers in Southern California.

In the case, which was brought by the Engine Manufacturers Association, The South Coast Air Quality Management District attempted to prohibit some fleet owners that operate in Southern California from buying many types of new vehicles, including diesel trucks and buses, that otherwise met all federal and California emissions standards and were certified for sale throughout California.

The district claimed its rule did not pre-empt the federal emissions standards – an act barred by regulatory rules – because it regulated the purchase of the vehicles, not the sale. However, the court ruled 8-1 that the action still pre-empted federal authority.

The Engine Manufacturers Association portrayed the decision as more far-reaching than just the vehicles and fleets it directly affects. The group said the court’s ruling means local agencies cannot adopt emission control programs that dictate the types of new vehicles that can be bought or sold.

“Today’s opinion clearly recognizes and affirms the intent of Congress to establish a national program governing emissions from mobile sources including trucks, buses, and cars,” Jed Mandel, President of the Engine Manufacturers Association, said in a statement.

However, despite the Supreme Court ruling, the Southern California district said in a statement that “the court left the door open for fleet rules” in some circumstances, and added that it would seek to continue to implement fleet rules in the areas it regulates.

“We are determined to continue implementing the rules for publicly owned fleets,” Barry Wallerstein, executive officer of the South Coast Air Quality Management District, said in a statement. “We will also consider asking the state and the U.S. Environmental Protection Agency to allow us to continue to regulate privately owned fleets.”