General Motors’ Opel division has been using defeat devices in diesel cars to bypass German emissions standards, according to a study cited by Germany’s Der Spiegel.

Researchers at Germany’s TÜV Nord inspection organization conducted tests that counter claims by Opel and by Germany’s Transport Ministry that “no defeat devices like in vehicles made by Volkswagen were found in cars by other makers.” Open had previously agreed to a recall in Germany along with Mercedes, Audi, Opel, Volkswagen and Porsche of 630,000 vehicles to fix their emissions management software.

During that time, Opel admitted that the exhaust treatment of a diesel-powered Opel Zafira would be fully operational only in a narrow temperature window “between 20 and 30 degree centigrade.” The new report from TÜV Nord found a number of other cases where Opel defeats its exhaust treatment.

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After conducting new tests an analyzing the engine computer’s code, the new report found the defeat device kicking in when the engine runs at higher revolutions than 2,400 rpm; when the Zafira goes faster than 145 km/h (90 mph); and when its barometric pressure is less than 915 millibar, indicating an elevation of more than 850 meters (2,788 feet).

A detailed report is expected to be published tonight in Germany’s ARD Monitor TV Magazine. Germany’s environmental pressure group Deutsche Umwelt Hilfe has scheduled a press conference for tomorrow to reveal its analysis of the exhaust performance of “Europe’s dirtiest Diesel car.”

In a response to Der Spiegel, Opel stated that there is no software “that registers whether the car is being tested for emissions.”