It's been months since the U.S. Environmental Protection Agency (EPA) revealed that Volkswagen had admitted it installed illegal "defeat device" software on its TDI diesel cars.

But the company still can't say when it plans to begin a U.S. recall of the affected cars.

And it looks like customers stuck with non-compliant diesels will have to keep waiting for a resolution.

DON'T MISS: VW Gets More Time To Figure Out TDI Diesel Emission Update (Dec 2015)

Volkswagen is still struggling with regulators to agree on a satisfactory fix, according to Reuters.

Citing an anonymous source, the news service claims relations between VW and regulatory agencies are "strained," but that the company still hopes to confirm a solution by a mid-January deadline.

Volkswagen submitted proposed modifications for the 482,000 2.0-liter TDI models confirmed to have "defeat device" software to the EPA and California Air Resources Board (CARB) in November.

2015 Volkswagen Golf TDI

2015 Volkswagen Golf TDI

It needs the approval of both agencies before it can proceed with a recall.

No details of the proposal have been made public, but it's thought that some cars may only require software changes, while others may need more extensive modifications--including the possible retrofit of Selective Catalytic Reduction (SCR) exhaust aftertreatment systems.

SCR systems use urea fluid injected into the exhaust stream to eliminate pollutants and require extra hardware, including a tank for the fluid, and plumbing.

ALSO SEE: How Will VW Fix My Diesel Car, And When? A List Of All Models (Oct 2015)

Installing these components could cost thousands of dollars per car, and push recall work into 2017.

Analysts--and some customers--have expressed skepticism over whether such expensive work is worthwhile, and argue VW should simply buy back many of the cars.

Attempting to modify cars is reportedly proving complicated, as the need to test new components has apparently slowed down the process of regulatory approval.

2010 Volkswagen Jetta TDI Cup Street Edition

2010 Volkswagen Jetta TDI Cup Street Edition

VW will hold further talks with EPA and CARB officials next week, according to the report.

The carmaker plans to begin a recall of certain diesel models this month in Europe, where the emissions standards it needs to meet are less strict.

MORE: Justice Department, EPA Sue VW Over Diesel Cheating, Billions In Penalties Possible

And while it continues to look for a solution for the U.S. cars, VW also faces a lawsuit from the Justice Department.

Filed in a Federal court in Detroit Monday, the suit alleges VW's use of "defeat device" software constitutes a violation of the Clean Air Act.

Penalties from the civil suit could add up to billions of dollars, and the Justice Department may pursue criminal charges as well.


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