VW has been embroiled in a scandal over emissions test rigging on its EA 189 engines.
It said it was now looking into older versions of EA 288 engines.
In September, Volkswagen said that 11 million diesel vehicles were affected by the scandal, first uncovered 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 €6.5bn to cover the costs of the scandal but many analysts expect this will not be enough.
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