Has Volkswagen installed a second computer software in its diesel vehicles that also affects the operation of its emission controls in its 2016 models in the US? That is exactly what the Federal and California regulators are investigating after Michael Horn, the head of VW's US operations disclosed that the company had withdrawn its 2016 models of its diesel car from environmental certification because of a computer software.

Horn had told a Congressional hearing on Thursday (9 October) that the company should have disclosed the software and sought regulators' approval before installing it. It had however failed to do both. The software qualifies as an auxiliary emissions control device which modifies the performance of emissions equipment.

The New York Times said that both Volkswagen and regulators have declined to say whether the second software was also intended to be used for the same purpose as the earlier cheat device. The German carmaker had admitted last month that 11 million diesel cars had been installed with a device that detects when the car is being tested for emissions. It then alters how the engine worked so as to produce lower emissions. However on open roads, the same vehicles were found to produce up to 40 times more nitrogen oxide than is legal in the US.

Stanley Young, a spokesman for the California Air Resources Board said the agency was investigating the nature of the second device but declined to provide specifics because VW's application "is still pending." He also could not say exactly when VW disclosed the existence of the device to the authorities. Young said that while the company had withdrawn its 2016 application with federal regulators, it has not withdrawn its application filed with Californian regulators.

"We're still going through all the information they provided and determining the next step," he told the New York Times. It said under normal circumstances, carmakers are allowed to use the auxiliary emissions control devices under deals negotiated with regulators that provide for higher emissions in very specific circumstances.

These could include when going up a steep hill or in very cold weather. These devices however must be disclosed to the authorities.

John German, a senior fellow specialising in emissions and efficiency technology at the International Council on Clean Transportation, the non-profit group that first noticed the real emissions from VW's diesel vehicles were far above regulatory limits said: "What we don't know yet is if the 2016 AECD is also a defeat device, or is a device that meets the guidance previously given by the EPA.

According to Dan Becker from the Safe Climate Campaign at the Center for Auto Safety, the second programme installed was a potential violation of the law. "They put it in their vehicles, and then they signed a certification petition to the regulator saying what they put in their vehicles and didn't mention it. It's certainly a material omission," he noted.