BERLIN (Reuters) - Germany's Transport Ministry has asked the European Commission to investigate exhaust emissions of Fiat Chrysler FCHA.MI vehicles for potential illegal manipulation devices, German government documents showed on Thursday.

Slideshow ( 2 images )

Germany's motor vehicle authority KBA began testing the vehicles of several manufacturers, including Fiat, after Volkswagen's VOWG_p.DE admission in September last year that it had cheated emissions tests with motor-management software.

The direct approach to the European Union executive comes after the German transport ministry raised concerns over Fiat vehicles with Italian authorities earlier this year and a subsequent rejection by Italian authorities of claims that Fiat and Chrysler vehicles used illegal exhaust manipulation devices.

This week’s letter to the European Commission, which was seen by Reuters, said that tests by German authorities on the Fiat 500X, Fiat Doblo and Jeep Renegade could prove the “illegal use of a device to switch off exhaust treatment systems” and urged the Commission to consult with Italian authorities to resolve the issue.

A Fiat spokeswoman said on Thursday that the company’s cars conform to current emissions rules and do not contain defeat devices.

The Commission, meanwhile, said that it is the responsibility of the Italian authorities to remedy wrongdoings.

“It is first and foremost a dialogue between the two member states concerned, with an obligation to keep the Commission informed and the possibility for the Commission to facilitate a solution if no agreement can be found,” the Commission said in a statement.

A source at the Italian infrastructure ministry, which includes the national motor vehicle authority, said Italy had not received any communication from the German government on the matter.

The source said Italian tests had shown Fiat 500 cars conformed to emissions rules and did not contain defeat devices, adding that the KBA had never said it disagreed with Italy’s findings.

As part of a widening clampdown on health-threatening nitrogen oxide (NOx) pollution levels in the wake of the VW scandal, the KBA tested 53 different vehicles and found that carmakers were making liberal use of what they described as a “thermal window”.

This refers to the time when carmakers are allowed to throttle back exhaust emissions management systems to protect engines from potential damage from condensation when cars are started in very cold conditions.

During their investigations, the KBA found that a very wide range of temperatures was used by carmakers for thermal windows.