Germany's transport ministry has summoned General Motors' European arm Opel to appear before an investigative committee, after several media reports revealed that the company is being suspected of emissions rigging.

Experts from TUV Nord of Germany said that Opel's diesel cars were found to have defeat devices. These devices are designed to bypass German emissions standards, based on a study cited by German magazine

Opel and the nation's Transport Ministry had claimed in the past that "no other vehicle has been found that has an illegal defeat device." Opel added that its software was "never designed to cheat or deceive."

Apart from Opel, other companies that had previously agreed to undergo a vehicle recall in Germany include Porsche, Volkswagen, Audi and Mercedes. Up to 630,000 vehicles are scheduled to get a recall in order to fix the issue on the emissions management software.

Spiegel Magazine further claimed that the defeat devices cause severely limited exhaust gas treatment in diesel cars, which would then result in poisonous NOx emissions that go beyond the allowable level. Experts believe this is illegal.

Opel 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.

Despite that, environmental group Deutsche Umwelt Hilfe (DUH) tested Opel's Zafira and Astra models and learned that the vehicles reduced pollution controls at certain speeds and temperatures. As a result, DUH believes that the vehicles should be taken off the road.

Opel, however, debunked the tests.

"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," said Opel in a statement. "Based on our own and independent measurements and on the experience with experiments published by DUH before, we do not believe that these results are objective or scientifically founded."

Opel also noted how the tailpipe emissions control systems seemed to be integrated and complicated. There are also a number of parameters that are involved and intertwined at the same time. These include temperature, altitude, rpm, load and engine speed.

Meanwhile, DUH scheduled a press conference where it will announce new defeat devices found in Opel's diesel cars.

Opel is scheduled to make an appearance before the investigative committee next week.

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