Should the engines have a so-called "defeat device", this could widen the number of vehicles affected.
VW has been embroiled in a scandal over emissions test rigging on its EA 189 engines.
It is now looking into older versions of EA 288 engines, although versions sold in Europe are not affected.
In September, Volkswagen said that 11 million diesel vehicles were affected by the scandal, first disclosed by the US regulator, the Environmental Protection Agency (EPA).
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.
VW 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.
The company 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 and in France and Italy, and VW has faced questions by MPs in the UK.
The firm has set aside â‚¬6.5bn to cover the costs of the scandal but many analysts expect this will not be enough.
Volkswagen says it is looking into more of its diesel engines to see whether they also contain software used to manipulate emissions test data.