The EPA maintained Harley-Davidson's Pro Super Tuner, which boosts a motorcycle's performance, also then allows a motorcycle to produce illegally high levels of emissions, including hydrocarbons and nitrogen oxides.
However, Harley-Davidson says the Pro Super Tuner has only ever been sold through its dealer network clearly marked 'for competition use only', and is therefore not for use on public roads.
Nevertheless, Harley-Davidson agreed to pay the EPA $US12 million in fines and contribute $US3 million to a clean air project to settle the case.
"This settlement is not an admission of liability but instead represents a good faith compromise with the EPA on areas of law we interpret differently, particularly EPA's assertion that it is illegal for anyone to modify a certified vehicle even if it will be used solely for off-road/closed-course competition," said Ed Moreland, Harley-Davidson's Government Affairs Director.
"For more than two decades, we have sold this product under an accepted regulatory approach that permitted the sale of competition-only parts. In our view, it is and was legal to use in race conditions in the US."
Harley-Davidson USA has said it will continue to sell "a performance tuner designed to ensure Harley-Davidson motorcycles retain 50-state and EPA on-road emissions compliance", while Harley-Davidson Australia has said the Pro Super Tuner will continue to be sold in Australia as competition-use-only product.
"Concern for our US customers and dealers weighed heavily in reaching this compromise with the EPA," said Moreland.
"By settling this matter, we can focus our future attention and resources on product innovation rather than a prolonged legal battle with the EPA."