A Volkswagen engineer has pleaded guilty to one count of conspiracy in the company's emissions cheating scandal and has agreed to cooperate in the widening criminal investigation.

James Robert Liang, of Newberry Park, California, entered the plea on Friday in US District Court in Detroit to one count of conspiracy to defraud the government through wire fraud.

Volkswagen has admitted to installing software on about 500,000 two-litre diesel engines in VW and Audi models in the US that turned pollution controls on during government tests and turned them off while on the road.

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The Environmental Protection Agency found that the cars emitted up to 40 times the legal limit for nitrogen oxide, which can cause human respiratory problems.

Liang was indicted in June on one count of conspiracy to commit wire fraud and another count of violating the Clean Air Act.

He allegedly helped design the computer software that cheated on diesel emissions tests. The indictment alleges Liang and unnamed co-conspirators resorted to using the cheating software after realising the cars could not both meet consumer expectations for performance and satisfy US emissions standards.

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Liang pleaded guilty to the conspiracy charge on Friday morning before Judge Sean Cox. He will be sentenced on January 11. The judge said that sentencing guidelines call for Liang to serve five years in prison.

The cooperation of Liang, who began work in Wolfsburg, Germany, and also worked in the US, is a major breakthrough in the Justice Department's investigation into the carmaker's cheating scandal.