By David Shepardson, Reuters.

A U.S. Environmental Protection Agency watchdog plans to review whether the agency's internal controls are effective at detecting vehicle emissions fraud, the EPA's Office of Inspector General said.

In a memo dated Monday, the inspector general said it will "begin preliminary research to determine whether the EPA’s existing internal controls are effective at detecting and preventing" light- and heavy-duty vehicle emissions fraud.

In September 2015, the EPA said it would review all U.S. diesel vehicles following an admission from Volkswagen that it installed software in 580,000 vehicles allowing them to emit up to 40 times the legally permissible level of pollution.

That extensive review prompted a delay in certification of some new diesel models last year.

VW sold vehicles with excess emissions for more than six years without EPA detecting the illegal software. VW, which is set to plead guilty on Friday as part of a plea agreement with U.S. prosecutors, has agreed to offer to buy back about 500,000 vehicles and agreed to spend up to $25 billion in the United States to address claims from owners, environmental regulators, states and dealers.