Volkswagen has admitted to US regulators it is using software in its 2016 diesel models that could potentially help exhaust systems run cleaner during tests.

The carmaker said that the "auxiliary emissions control device" at issue operates differently from the "defeat" device software included in the company's 2009 to 2015 models disclosed last month.

The new software was first revealed to Environmental Protection Agency (EPA) and California regulators on September 29, prompting the company last week to withdraw applications for approval to sell the 2016 cars in the US, reports the Associated Press.

"We have a long list of questions for VW about this," said Janet McCabe, acting assistant EPA administrator for air quality. "We're getting some answers from them, but we do not have all the answers yet."

The delay means that thousands of 2016 Beetles, Golfs and Jettas will remain quarantined in US ports until a fix can be developed, approved and implemented.

Diesel versions of the Passat sedan manufactured at the company's plant in Chattanooga, Tennessee, also are on hold.

However, Volkswagen UK has informed Fleet News that the software is only being deployed in US models and does not include new Euro 6 models destined for the UK. 

The software at issue makes a pollution-control catalyst heat up faster, improving performance of the device that separates smog-causing nitrogen oxide into harmless nitrogen and oxygen gases.

"This has the function of a warmup strategy which is subject to approval by the agencies," said Jeannine Ginivan, a Volkswagen spokeswoman. "The agencies are currently evaluating this and Volkswagen is submitting additional information."

Automakers routinely place auxiliary emissions control devices on passenger vehicles, though they are required by law to disclose them as part of the process to receive the emissions certifications that are required to sell the cars.

EPA's McCabe wouldn't say if Volkswagen’s failure to disclose the software in its 2016 applications was illegal. "I don't want to speak to any potential subjects of an enforcement activity," she said.

Volkswagen of America CEO Michael Horn said in congressional testimony last week that the German automaker had withdrawn applications seeking certification of its 2016 diesels because of on-board software that hadn't been disclosed to regulators.

However, Horn's statement left unclear whether the issue with the 2016 models was the same as that in the earlier models, or whether it potentially constituted a new violation.

A congressional staffer briefed on the issue told AP that VW probably didn't need the additional software to meet government emissions standards, but that the device appears intended to ensure the 2016 cars would pass inspection by wider margins. The staffer spoke on condition of anonymity because he was not authorized to talk publicly about the ongoing investigation.

VW is now working with regulators to continue the certification process needed to sell its 2016 diesel cars.

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