Volkswagen says its 2016 diesel models have suspect software that could help exhaust systems run cleaner during government tests.
Europe’s largest car manufacturer said the issue involved the vehicles’ “auxiliary emissions control device”.
It operates differently from the so-called “defeat” device that last month triggered the cheating scandal engulfing the carmaker.
VW said the latest matter was revealed last week to the US Environmental Protection Agency and California regulators.
The software makes a pollution-curbing catalyst heat up faster to enhance performance of the device that breaks down smog-causing nitrogen oxide.
VW spokeswoman Jeannine Ginivan said: “This has the function of a warm-up strategy which is subject to approval by the agencies.
“The agencies are currently evaluating this and Volkswagen is submitting additional information.”
In another blow to the carmaker on Wednesday, the man Volkswagen lined up less than three weeks ago to head its North American business resigned.
Company veteran Winfried Vahland stepped down as Germany’s Spiegel magazine reported that at least 30 VW managers were involved in the test cheating.
Last week, the head of Volkswagen’s US business, Michael Horn, said only “a couple of software engineers” were responsible.
The German carmaker has been in turmoil since admitting last month its 2009 to 2015 models were using a “defeat” device to hide their toxic emissions from US regulators.
The controversy has battered VW’s share price, forced out its chief executive and prompted investigations around the world.