Volkswagen has confirmed there is additional suspect software in its 2016 diesel models that would potentially help their exhaust systems to run cleaner during US emissions tests.

A VW spokeswoman said the new issue was first revealed by the firm last week to the US Environmental Protection Agency. Regulators have not yet determined whether the code is a defeat device installed specifically to cheat on emissions tests, said the EPA's Janet McCabe.

"We have a long list of questions for VW about this," she said. "We're getting some answers . . . but we do not have all the answers yet." Volkswagen already faced an ongoing criminal inquiry and billions in fines. In further moves inside the group, Skoda chief Winfried Vahland – previously due to be appointed as new head of VW North America – has opted to leave the firm instead.

There was at least some good news for the car giant yesterday as German officials announced plans to subsidise the purchase of electric cars, a day after VW announced plans to increase its production of electric and hybrid models.

The federal environmental ministry has decided that, to help electric cars break into the mainstream, it must offer “alongside tax incentives a combination of obligatory quotas for e-vehicles, combined with state purchasing subsidies”.