Volkswagen is investigating whether cars using early versions of its EA 288 diesel engine may have contained software that could manipulate emissions test data.
Euro 5 engines of this series, which is different to the first affected model, could have had the software installed but it is not in the Euro 6 versions.
It has established about a quarter of the 11 million global recalled vehicles may need a hardware change as well as software modification and the company now believes the total cost of the corrective campaign will be the most expensive in automotive history when adding any civil claims to the physical change costs.
VW will need to install parts for vehicles already on the road that were not designed to accommodate the equipment.
The work may need to be done in special centres set up for the purpose and the changes it will have to pass dozens of countries with their own regulations.
It has had to stop selling some new cars thought to be fitted with the deceit devices, compensate dealers and pay storage charges and is facing over 300 US lawsuits.
German management experts have put the total cost to Volkswagen at €30 Billion or £22 Billion.