U.S. and California regulators are investigating whether a second device designed to foil emissions regulations has been installed in some Volkswagen models, according to the New York Times.

A spokesman for the California Air Resources Board admitted to the newspaper that it is looking into Volkswagen's certification documents and the impact of a second device. The Environmental Protection Agency is also investigating.

The auxiliary emissions control device would be in addition to the engine software exposed last month that switched on emissions controls during testing conditions but turned them off during real-world driving.

The California regulator has given Volkswagen until Nov. 20 to come up with a fix for its diesel engines that spew too much nitrogen oxide.

On Thursday, the CEO of VW's U.S. operations Michael Horn presented background documents to Congress that showed Volkswagen had withdrawn its 2016 diesel cars from environmental certification over a software device.

The International Council on Clean Transportation, the research group that first exposed Volkswagen's emissions cheating, has noted the presence of an auxiliary emissions control device on 2016 models.

Automakers are allowed to use such devices to allow higher emissions when a vehicle is going uphill or in very cold weather if they are approved by regulators.

But Horn testified Thursday before a House energy and commerce subcommittee that the company had neglected to disclose the software and seek approval from regulators.

The company withdrew documents for its 2016 Jetta, Passat, Beetle and Golf diesel cars from review by emissions regulators, meaning it could be months before new VW diesel models are released into the market.

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.

As a result of this device, the so-called "clean diesel" vehicles were found to produce up to 40 times more nitrogen oxide than is legal under U.S. emissions regulations.

Horn was pressed by U.S. lawmakers Thursday to say who was responsible for making the decision about the device, which has been in cars since 2008.

About 100,000 Canadian cars are affected.