Daimler boss Dieter Zetsche will have to please explain to the German minister of transport. File photo: Markus Schreiber / AP

Frankfurt, Germany - Daimler boss Dieter Zetsche has been summoned to meet transport minister Andreas Scheuer after Germany's KBA motor vehicle authority discovered “inadmissible defeat devices” in Mercedes-Benz engines. German weekly Bild am Sonntag reported on Sunday that as many as a million cars could contain software to cheat emissions tests on diesel vehicles.

All carmakers use software to manage exhaust emissions filtering and engine performance, but a device can be classified as illegal if exhaust filtering systems are deactivated too early or without good reason. Daimler, like other car manufacturers, uses urea nitrate liquids to neutralise nitrogen oxide emissions in exhaust fumes, but the KBA is taking issue with the emission control features amid suspicions that they allow vehicles to emit excess pollution without detection, although Daimler insistes the software is legal.

The company won't comment on the figure of a million cars but says it is cooperating fully and transparently with the KBA and Germany’s transport ministry. The emissions scandal has hung over the German car industry since September 2015, when Volkswagen admitted to using software that could tell when a diesel vehicle was being tested and temporarily lower its toxic emissions to pass US regulations.