German carmaker Opel went on the defensive Friday, laying out arguments explaining how its technology is effective at reducing auto emissions and pushing back at recent criticism that its diesel cars put out more pollutants than advertised.

Allegations that Opel manipulated emissions data prompted the German Transportation Ministry last week to launch a closer inspection of the business' practices and for the legislative opposition in the Bundestag to empanel their own probe.

Unlike Volkswagen - which has admitted to installing software to cheat emissions tests - Opel has batted back all charges it uses illegal software, and says it has used no measures to spoof its emissions reports. It has said that any reduced performance registered is due to technical reasons.

The concerns about Opel's cars surfaced after environmental pressure group DUH alleged that the system to scrub emissions from the company's Zafira diesel model only worked 80 per cent of the time.

But Opel said the reports questioning its environmental credentials grossly oversimplified how modern cars work and did not take enough variables into account.

The company noted that a modern motor control unit includes more than 17,000 parameters and any review of the engine has to consider the interplay between them. Picking out individual parameters will provide a false view of what's really happening, said the company.

Opel said that DUH's arguments that it's diesel Zafira model's emissions cleansing system only worked 80 per cent of the time was simply wrong. It argued that the system remains active when the outside temperature falls below 17 degrees Celsius.

It also said charges that the system turns off when the car reaches more than 2,400 revolutions per minute, saying that what really happened was that the car switched to a different mode.

However, it did note that the system does work at lower capacity when the car is moving at high speeds, a function necessary to keep poisonous ammonia from being emitted. It noted a similar problem when its cars are used in high altitudes with low air pressure, because the system cannot function properly in low-oxygen environments.

But it said that problem had already been addressed, meaning it would not occur until the car was driven at an altitude of 1,500 metres. Currently, the cut-off is 900 metres.

The concerns about Opel feed off a series of allegations - starting with concerns about Volkswagen - that have raised concerns about whether diesel cars are capable of meeting Western environmental standards.

Germany's Transport Ministry has conducted tests showing that 30 car models out of 53 tested emit more carbon dioxide than expected, meaning fuel consumption rates are higher than advertised. The results have prompted the ministry to expand its tests of compliance with environmental standards.

A ministry spokeswoman did not provide details about the nature of the test or say when a report would be released.

The tests had been designed to check emissions for nitrogen oxide, but the carbon dioxide values noticed during testing forced a closer look.

Opel said Friday that it has no plans to give up on diesel cars because such cars have significantly lower carbon dioxide emissions and fuel consumption than other kinds of cars.

In the first quarter of 2016, 37 per cent of Opels sold were diesels. Other carmakers sell much higher percentages of diesel cars.

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