VW has announced it is looking into more of its diesel engines to see whether they also contain software used to manipulate emissions test data.
The German automaker has been embroiled in a scandal over emissions test rigging on its EA 189 engines.
VW is now looking into older versions of EA 288 engines, although versions sold in Europe are not affected.
The EPA discovered that certain engines were fitted with a cheat-device that could detect when the car was being tested and reduced emissions to improve results.
Volkswagen had previously said that the software was installed on cars with variants of the EA 189 diesel engine built to the “Euro 5” emissions standard.
VW is now checking whether models with the EA 288 diesel motor built to the same emissions standard may also have the software.
Newer EA 288 cars built to the “Euro 6” standard are said not to be affected.
Criminal investigations have been launched in Germany, France and Italy.
VW has set aside €6.5 billion to cover the costs of the scandal but many analysts expect this will not be enough.