California regulators recently discovered software installed on some of Volkswagen's Audi models appears to have allowed the cars to cheat carbon dioxide emissions testing standards, Wall Street Journal (WSJ) sources said.

The Audi software was designed to mask emissions implicated in global warming, instead of smog as in the Volkswagen emissions cheating scandal that erupted last year, the sources said.

The newly discovered software was detected four months ago during laboratory tests by the California Air Resources Board, one source told the WSJ which noted neither Volkswagen nor US regulators had publicly disclosed the discovery.

Officials at CARB, which has been heavily involved in a continuing US probe of Volkswagen engines, didn't respond to WSJ requests for comment while Audi declined to comment, citing the Justice Department investigation into the Volkswagen scandal.

The WSJ said it wasn't clear how seriously officials in California and federal officials in Washington view the latest discovery, or whether Volkswagen, under intense regulatory scrutiny around the world, had identified it privately to regulators.

Previously disclosed 'defeat device' software used on Volkswagen and Audi diesel engines made it appear that they complied with emission standards for nitrogen oxides during lab tests and the newly discovered software, installed on Audis with both diesel and petrol engines, did the same with CO2 emissions standards in the US and Europe, according to the Wall Street Journal sources "familiar with the matter".

CARB caught the emissions cheating software through lessons learned from the earlier probe of Volkswagen diesel engines, according to German weekly Bild am Sonntag which earlier reported the software's discovery.

CARB technicians conducting lab tests on Audi's vehicles made them react as if on a road by turning the steering wheel, the sources told the WSJ. When the cars deviated from lab conditions, their CO2 emissions rose dramatically.

Volkswagen said on Sunday that a German criminal investigation related to the diesel emissions scandal has widened to include its chairman.

Volkswagen and Audi management discussed the CO2 defeat-device software in detail during a 'Summer Drive' event in South Africa in the second half of February 2013, according to one person familiar with the situation and excerpts from the minutes of the meeting, which were reviewed by the Wall Street Journal.

According to the minutes, Axel Eiser, head of Audi's powertrain division, said, "the shifting program needs to be configured so that it runs at 100% on the treadmill but only 0.01% with the customer."

Audi declined to make Eiser available to the WSJ for comment.

The paper said it wasn't clear which Audi models might contain the newly discovered software, which could raise fresh questions in Europe, where regulators have been stricter on emissions of greenhouse gases like CO 2 than on nitrogen oxides.

Volkswagen insists that its software didn't violate European law. In Germany, Volkswagen hasn't been charged with violating the law, the WSJ added.