Opel has refuted claims in Germany that it has so-called 'defeat devices' installed to some of its diesel engine models in order to rig emissions tests.

Germany's Transportation Ministry announced last week that it will look into the business practices of Opel, following allegations by environmental activists and media outlets that Opel's diesel models have been cheating emissions tests.

The investigations will be carried out by a special commission that was established in the wake of the Volkswagen Dieselgate scandal. It has asked representatives from Opel – as well as those from Italian carmaker Fiat, which is also under inspection – to meet with the ministry.

Environmental activist group Deutsche Umweltilfe (DUH) in cooperation with German news organisation ARD and magazine Der Spiegel says that Opel's Astra and Zafira models were producing "alarming" amounts of emissions and were equipped with "kill switches" that hid this fact from testers.

Experts from TUV Nord of Germany also said that Opel's diesel cars were found to have defeat devices. According to Der Spiegel Magazin, these defeat devices cause "the exhaust gas treatment in those car to be severely limited, allowing the emissions of more poisonous NOx than permissible by law. Experts say this is illegal."

Opel has admitted that the exhaust gas treatment of the diesel-running Opel Zafira would become fully operational within a narrow temperature window ranging between 20 and 30 degrees Celsius. However, the testing and certification organisation TUV Nord now reportedly has found a number of other cases where Opel defeats its exhaust treatment. According to the report, new tests, and an analysis of the engine computer's code reveal that the exhaust treatment of the Opel car are switched off when the engine runs at higher revolutions than 2400 rpm, when the car is travelling at more than 90mph and when the barometric pressure is less than 915 millibar.

Opel says its software was never designed to cheat or deceive. In a statement, the company said that "we do not deploy any software that recognizes whether a car is undergoing an exhaust emissions test". It also said the German Federal Ministry of Transport and Digital Infrastructure has concluded that – "apart from certain cars of a competitor" – no other vehicle has been found that has an illegal defeat device. Commissions in France and the United Kingdom already came to the same conclusion, it adds.

Further, Opel maintains that the test methods and protocols of the activities by DUH/Monitor/Spiegel were not shared with Opel "which does not allow us to evaluate the outcome" and says it does not believe that the results are objective or scientifically founded.

Opel also points out that "tailpipe emissions control systems are complicated and integrated" and says "multiple parameters, e.g. engine speed, load, rpm, altitude and temperature, play a role and are intertwined".

It adds: "This complex system cannot be cut into single parameters. The interaction is to be understood holistically in combination of conditions and elements of the control system."