Volkswagen has informed Germany’s transport authority that half a million cars recalled for a fix after the diesel scandal may still be equipped with illegal software designed to trick emissions tests.

The German carmaker, which has spent more than €28 billion on penalties and car repurchases since US authorities disclosed in late 2015 that it had cheated emissions tests for a decade, said that its quality control department discovered a new problem in which certain cars might still limit emissions when the vehicle starts up.

Volkswagen said the issue concerns 1.2-litre, EA-189 engines. There are roughly 500,000 such cars globally, including 32,000 in Germany. VW estimates about three-quarters of these have been fixed with the software update in question.

As a matter of precaution, VW said the update “should initially be suspended,” to avoid any legal trouble in case it turns out to be illegal.

Two people at VW stressed that the issue, first reported by Bild am Sonntag, could amount to nothing or be in a legal grey area, but VW has become ultra-sensitive to any potential legal issues and so it informed the authorities immediately.

Both people noted VW has been criticised in the past for sitting on material information regarding the diesel scandal.

In this case the potential software problem was discovered last week. Because of the holidays, VW is unable to look closely at the problem until early January. It was determined that the optics of there being a three-week delay between detection and analysis could be poor, so VW informed the German transport authority, known as the KBA.

One person said the KBA could give four answers, ranging from saying the software is fine to deeming it an illegal defeat device. A recall of the cars is possible, either to fix the cars as soon as possible or at their next routine service update.

- Copyright The Financial Times Limited 2018