A Windham man has pleaded guilty to conspiring to tamper with diesel truck monitoring devices intended to enforce the federal Clean Air Act and obstructing efforts to detect his actions, the U.S. attorney’s office said Thursday.

Isaac Allen, owner of Red Barn Diesel Performance, faces up to five years in prison and a maximum fine of $250,000, or twice the gross gain or loss from the offense. He entered the plea Wednesday in U.S. District Court in Portland.

Tyler Amon, special agent in charge of the U.S. Environmental Protection Agency’s criminal investigation division, said Allen illegally tampered with the vehicle diagnostic systems from diesel trucks and impeded EPA’s investigation by intentionally misrepresenting the scope of his illegal conduct. Pollution from excess vehicle emissions can lead to health hazards and has been linked to increased respiratory disease and childhood asthma, he said.

“Isaac Allen has done what we want someone who has made a mistake to do: He has admitted his mistake,” his lawyer Timothy Zerillo said in an email. “He has pleaded guilty to a violation of the federal Clean Air Act and he did so expeditiously.” Zerillo also said Allen had no prior criminal record.

Prosecutors say Allen, 34, conspired with a local truck sales business to download software that reprograms a vehicle’s on-board diagnostic system between January 2017 and September 2020, according to the U.S. attorney’s office. Interfering with the on-board system’s monitoring function would prevent it from detecting malfunctions in emissions control components.

Disabling emissions controls or tampering with the on-board diagnostic system of a diesel truck causes its emissions to increase significantly. Diesel exhaust contains nitrogen oxides, carbon monoxide, particulate matter and other pollutants that are health hazards.

Prosecutors also say Allen underreported information sought by the EPA about the vehicles serviced by Red Barn, including identification of all engine “tunes,” the reprogramming software, and a description of whether the tunes affected various components of each vehicle’s emissions control system or interfered with each vehicle’s on-board diagnostic system.