Opel said that its software had no features that would enable emissions treatment technology only under test conditions.
It was responding to allegations from Deutsche Umwelthilfe (DUH) that its cars had a different emissions profile when tested with and without the rear wheels moving.
"The measurements show that the vehicle behaves differently when the rolling test bed is operated in four-wheel or two-wheel mode," testers from the University of Applied Sciences in Bern, Switzerland, said.
DUH had asked the emissions specialists in Bern to test the NOx emissions of an Opel Zafira car with a 1.6-litre diesel engine, supposedly designed to comply with the new, tougher Euro-6 emissions regulation from 2014.
They found that when the car was put through the official testing cycle known as the NEDC with its rear wheels turning, it emitted two to four times more NOx than permissible.
But when the rear wheels were prevented from turning – as happens under normal NEDC test conditions – the emissions were lower than the legal limit of 80 milligrammes per kilometre.
By the time the vehicle was operating at a speed of 150 km/h, Nox emissions rose sharply beyond the levels measurable with the equipment on hand.
"This behaviour could be explained by the switching off of the AdBlue [fluid sprayed into exhausts to neutralise NOx gases] injection system," the Swiss report read.
Charity was 'tipped off'
"We decided to start with the Zafira in our emissions tests because we had detailed tip-offs about inconsistencies in the emissions treatment system in this vehicle," DUH manager Jürgen Resch said in a statement.
"We have sent the testing report to the Federal Motor Vehicle Authority (KBA) today and asked them to test the model themselves."
DUH lawyer Remo Klinger said that the KBA had yet to provide the green charity with detailed information about its orders to Volkswagen following that car-maker's diesel emissions scandal.
Opel rejects allegations
Opel, which is owned by US auto giant General Motors (GM), released its own statement on Friday saying that "Opel completely rejects the assertions of DUH.
"It is true of all our cars, without limitations: software developed by GM has no features that detect whether the vehicle is being subjected to an emissions test."
The company said that DUH had refused to make the results of the testing carried out in Bern available to them for study.
But they added that Opel engineers had done their own tests with an identical car, with the test-bed in both two- and four-wheel mode, and that "the values fell within the legal regulations in both two- and four-wheel measurements.
"This must be the case, because there is no effect on the exhaust treatment system whether or not the Zafira's rear wheels are turning.
"The allegations are simply false and groundless."