Germany's Volkswagen, already reeling from news that it had cheated on U.S. tests for nitrogen oxide emissions, said Tuesday that an internal investigation had revealed "unexplained inconsistencies" in the carbon dioxide emissions from 800,000 vehicles.

The revelation comes after VW's admission in September that it rigged emissions tests for four-cylinder diesel engines on 11 million cars worldwide, including almost 500,000 in the U.S. It has already set aside 6.7 billion euros ($9.5 billion Cdn) to cover the costs of recalling those vehicles and these additional problems are projected to cost $2.8 billion. 

It was not immediately clear whether the 800,000 vehicles with the newly discovered carbon dioxide emission problems were among those already affected. Volkswagen did not identify which models were affected.

Scandal widens to gas engines?

It did say the 800,000 were "predominantly vehicles with diesel engines," raising the possibility for the first time that some Volkswagens with gasoline-powered motors may also have emissions problems.

Carbon dioxide is a greenhouse gas that causes climate change and regulators set limits on how much can be emitted by automobiles.

A VW spokesman did not immediately return a phone call seeking clarification.

CEO Matthias Mueller, who took over from Martin Winterkorn, who resigned because of the emissions scandal, promised in a statement that Volkswagen "will relentlessly and completely clarify what has happened."

"It is a painful process but for us there is no alternative," he said. "For us only one thing counts, and that is the truth."

The investigation that revealed the inconsistencies in the carbon dioxide emissions was undertaken by Volkswagen after the revelations of its cheating on U.S. nitrogen oxide testing.

The company assured customers that the safety of the vehicles in question "is in no way compromised."

It said Volkswagen "will endeavour to clarify the further course of action as quickly as possible and ensure the correct CO2 classification for the vehicles affected" with the responsible authorities.

In talks with the authorities - whom Volkswagen did not identify - the company said it hoped to come up with a "reliable assessment of the legal, and the subsequent economic consequences of this not yet fully explained issue."

The news broke after Germany's DAX was closed for the day.

With files from CBC News