The trial for a class-action lawsuit against Mercedes-Benz began Tuesday over a lawsuit that alleges the German carmaker knowingly manipulated diesel-emissions tests by installing defeat devices.

VZBZ, Germany's largest consumer protection group, has accused the carmaker of installing devices in its GLK and GLC SUV models that made it appear the vehicles produced lower pollutant levels in tests than they actually did.

The Stuttgart court must decide whether Mercedes-Benz knowingly deceived customers by controlling the purification of exhaust gas, thus meriting claims for damages. The lawsuit seeks to set a precedent that would enable owners of Mercedes GLC and GLK cars to gain compensation for the software allegedly used to pass emissions tests.

The lawsuit covers nearly 50,000 GLC and GLK models. The challenge became possible after Germany passed a law in 2018 that allowed consumer protection organizations to litigate on behalf of the consumers they represent.

Mercedes-Benz says the claims leveled by diesel customers as well as the lawsuit are unfounded. Over 25,000 such claims have been brought before courts, 95% of which have failed, the automaker said.

The allegations are part of a larger “Dieselgate” emissions scandal that has cost Volkswagen billions of euros in vehicle refits, fines and legal costs.