The California Air Resources Board (CARB) says Volkswagen’s proposed plan to fix 3.0-liter diesels is not good enough.

Regulators in California have called on Volkswagen to come up with a better fix for 16,000 cars with 3.0-liter diesel engines which were illegally fitted with defeat devices to cheat emissions tests. Those include models from the VW, Audi and Porsche.

“VW’s and Audi’s submissions are incomplete, substantially deficient, and fall far short of meeting the legal requirements to return these vehicles to the claimed certified configuration,” CARB said in a letter.

CARB said that it would need until December to determine whether an acceptable fix was even possible for all 3.0-liter diesel vehicles affected.

That leaves a lot of uncertainty for the VW Group. If an acceptable fix is not possible, that would mean VW would have to buy back the cars and add billions in costs to its current settlement. Last month, VW reached a settlement with U.S. authorities that will cost the automaker up to $15.3 billion, though that only covers cars fit with 2.0-liter diesel engines. That agreement included a buy back scheme for up to 475,000 2.0-liter diesel cars.

VW said through a spokesperson that it will continue to work with the EPA and CARB to find a solution to the problem. An Audi spokesman also downplayed the setback, describing the rejection of the fix as a “procedural step under California state law.”

The V6 3.0-liter diesel unit in question was designed by Audi.

Reuters

This article originally appeared at AutoGuide.com