The results of the nitric oxide tests carried out by environmental group Deutsche Umwelthilfe (DUH) and German state broadcaster ZDF - who broadcast it on December 15 - appeared to show similar discrepancies between "test mode" and road conditions to those found in Volkswagen cars earlier this year, revelations that triggered one of the biggest scandals in German automobile history.

In response to the report, a law firm representing Daimler, which owns Mercedes, sent a letter to the DUH that read, "Should you in any way present the accusation that my client manipulated its emissions data, we will act against you with all necessary sustainability and hold you responsible for any economic damage that my client suffers as a result."

The law firm, called Schertz, also warned the DUH against publishing the letter itself, but the group defied this and posted the letter in full on its website.

"We have been massively threatened two more times, demanding that we take down the letter - we have told them we won't," DUH chairman Jürgen Resch told DW on Wednesday. "For me it's a very serious issue, because in 34 years of full-time work in environmental protection, and dealing with businesses, I have never experienced a business using media law to try and keep a communication - and a threatening letter at that - secret.

VW boss Martin Winterkorn was forced to resign as a result of the emissions scandal

"How are we supposed to do our work as a consumer and environmental protection organization when industry forbids us from making public certain threats it makes?" an outraged Resch added. "I think the threat itself is borderline legal coercion."

Dirty diesel - on the road

In the documentary, ZDF tested three diesel cars - a Mercedes C200 CDI from 2011, a BMW 320d from 2009, and a VW Passat 2.0 Blue Motion from 2011 - and showed that all three produced several times more nitric oxide on the road than they did in an official laboratory test.

"The measurement results show that the cars behave differently on the test dynamometer than when they are driven on the road," said the laboratory at the University of Applied Sciences in Bern, Switzerland, which carried out the tests.

The discrepancies were not small. While all three cars kept comfortably below the European Union's legal nitric oxide limit (180 milligrams per kilometer) in the lab, out on the road the BMW recorded 428 mg/km (2.8 times its lab result), the Mercedes hit 420 mg/km (2.7 times its lab result), and the VW Passat reached 471 mg/km (3.7 times its lab result).

No technical explanation

Daimler did not respond to a request for comment from DW, but company spokesman Jörg Howe told the "taz" newspaper that the company could not verify the test result and pointed out that the cars had already been used.

"No one can rule out that they weren't damaged or manipulated by a third party," he said. He also said outside temperatures, road surfaces, and wind could all have played a part in the discrepancy. A spokesman for BMW denied to ZDF that it used any kind of manipulative device.

These answers did not wash with the DUH or with other experts.

"I would expect discrepancies in the nitric oxide measurements, but not on this scale," Kai Borgeest of the Center for Automobile Electronics and Combustion Engines at Aschaffenburg University told ZDF. "Technically it is conceivable that the other manufacturers tested used 'power down' installations in different ways - in other words software functions that recognize the test cycle. That would be illegal."

Resch of the DUH went even further.

"Neither BMW nor Mercedes can explain how these massive discrepancies came about," he told DW. "Talking about the wind or that there were more people in the car could maybe explain a 3 percent difference - we've got 300 percent. These discrepancies cannot be technically justified."

Government silence

Amidst this new scrutiny, the German government is maintaining a stony silence. In response to a DW request, the Transport Ministry did not offer any other explanation for the test discrepancies.

"The federal automobile authority [KBA] is currently carrying out tests on the affected Volkswagen diesel models as well as other major manufacturers of diesel cars ... the tests are taking place both on the 'roller' and on the street," the ministry said in a brief statement.

The German government is being urged to do better emissions testing

The KBA said it could not comment as its press spokesman was not in the office.

This lack of explanation is beginning to irk environmental groups.

"We haven't heard anything from the KBA about Mercedes. We haven't even got a confirmation from the German authorities that they have received our report," said Resch. "Interestingly enough - the EU Commission, the European Parliament, and some foreign governments are all highly interested in our tests.

"And we haven't had to take anything back, content-wise," he added. "In all the years, every single line of our statements are all still there unchanged - and you can bet that Renault, BMW, VW, Mercedes did everything they could to find a false statement or to claim that our measurements were wrong. They haven't done that."

The new battle with Daimler is just the latest in the DUH's long campaign to try and get the German government to control emissions from carmakers.

"In February 2011, we named the VW Passat with extremely high nitric oxide emissions - that was whistleblower information that, of course, we could only show to the authorities," he said. "For over eight years, every year we've made a scandal of it. That's why we desperately need federal agencies that have the courage to investigate these things.

"Now we're handicapped by this existence-threatening pressure - because a lawsuit would mean our insolvency," Resch said. "That does make me pretty angry, to be obstructed like this."