German weekly Bild am Sonntag said up to 1 million cars contained software to cheat emissions tests on diesel vehicles.
Carmakers use software to manage exhaust emissions filtering and engine performance. A device can be classified as illegal if exhaust filtering systems are deactivated too early or without good reason.
At 0836 GMT, Daimler shares were trading 1.76 percent lower, at 61.19 euros, underperforming the Stoxx 600 Automobile index which was 0.48 percent lower.
German Transport Minister Andreas Scheuer summoned Zetsche after the country’s KBA Motor Vehicle Authority discovered “inadmissible defeat devices” in Mercedes-Benz engines, even as Daimler disputes they were illegal.
Daimler declines to comment on the 1 million vehicles figure, but said it is cooperating fully and transparently with the KBA and Germany’s Transport Ministry.
Daimler, like other car manufacturers, uses urea nitrate liquids to neutralise nitrogen oxide emissions in exhaust fumes. However, Germany’s road vehicle authority, the KBA, has taken issue with the emission control features amid suspicion they allow vehicles to emit excess pollution without detection.