Emissions from certain vehicles were 10 percent more on average than GM’s initial compliance testing claimed.

The National Highway Traffic Safety Administration said in a statement Wednesday that certain GM vehicles from the 2012 through 2018 model years did not comply with federal fuel economy requirements.

The fine came after the Environmental Protection Agency said its testing showed the GM pickup trucks and SUVs emit 10 percent more carbon dioxide on average than GM’s initial compliance testing claimed.

The EPA says the vehicles will remain on the road and cannot be repaired.

GM said in a statement that it complied with all regulations in pollution and mileage certification of its vehicles. The company said it is not admitting to any wrongdoing or that it did not comply with the Clean Air Act.

The enforcement action involves about 4.6 million full-size pick-ups and SUVs and about 1.3 million midsize SUVs, the EPA said. The affected models include the Chevy Tahoe, Cadillac Escalade and Chevy Silverado. About 40 variations of GM vehicles are covered.

GM will be forced to give up credits used to ensure that manufacturers’ greenhouse gas emissions are below the fleet standard for emissions that applies for that model year, the EPA said.

An EPA spokesman said the violations were unintentional.

But David Cooke, senior vehicles analyst for the Union of Concerned Scientists, questioned how GM could not know that pollution exceeded the initial test by more than 10 percent because the problem was so widespread on so many different vehicles. “You don’t just make a more than 10 percent rounding error,” he said.

He also was surprised that the government is not making GM take action to correct the excess pollution or reduce the gas mileage estimates on the cars’ window stickers.

Dan Becker, director of the Safe Climate Transport Campaign for the environmental group Center for Biological Diversity, said the violations by GM “show why automakers can’t be trusted to protect our air and health, and why we need strong pollution rules. Supreme Court, take notice!”

The Supreme Court last week rejected a 40-year-old legal doctrine known as Chevron, effectively reducing the power of the EPA and other executive branch agencies and shifting it to the courts. The doctrine has been the basis for upholding thousands of federal regulations but has long been a target of conservatives and business groups, who have argued it grants too much power to the executive branch, or what some critics call the administrative state.

In similar pollution cases in the past, automakers have been fined under the Clean Air Act for such violations, and the Department of Justice normally gets involved, Cooke said. Hyundai and Kia, for instance, faced Justice Department action in a similar case.

A Justice Department spokesman did not have an immediate comment Wednesday.