The U.S. Environmental Protection Agency will implement additional testing of diesel vehicles designed to uncover potential software algorithms used to evade federal emissions standards, the agency announced Sept. 25.

The agency sent a letter to all manufacturers informing them about the expanded testing protocol following the revelations that Volkswagen had implemented software that turned on a vehicle's full emissions controls only during testing.

The Sept. 25 letter informed manufacturers that they could expect the additional tests in addition to the standard emissions test cycles during Emissions Data Vehicles (EDV) and Fuel Economy Data Vehicles (FEDV) tests.

On Sept. 18, the EPA accused Audi and Volkswagen of using a software algorithm in its four-cylinder diesels to circumvent federal emissions standards. The cars from the 2009 to 2015 model years could detect when the car is undergoing official emissions testing and turn on full emissions controls only during that test. This would violate the Clean Air Act.

The allegations cover about 11 million models, such as the Jetta TDI (2009-2015), Jetta SportWagen (2009-2014), Beetle TDI (2012-2015), Golf TDI (2010-2015), Golf SportWagen (2015), Passat TDI (2012-2015) and Audi A3 TDI (2010-2015).