The California Air Resources Board (CARB), which works closely with the federal EPA on automotive emissions issues, announced today that it has rejected Volkswagen's US proposal for fixing cars affected by the company's global diesel cheating scandal. The proposal had been submitted in late November.

This specific rejection only affects 2-liter diesels, the most common engine tied up in the scandal. The proposal for fixing 3-liter models, which are also affected, is not due for evaluation by CARB until next month.

This is undoubtedly a huge blow for Volkswagen, which has been desperate to rapidly clean up the mess affecting millions of vehicles around the world that was first made public in September of last year. In the months since, it has reshuffled executives, announced a variety of fixes in various markets, dialed down its diesel marketing (and suspended diesel sales altogether in some parts of the world), and refocused on electrics with the introduction of the Budd-e concept and the greenlighting of the Porsche Mission E concept.

In its press release, CARB cites three reasons for the rejection: "gaps and a lack of sufficient detail," a lack of "enough information for a technical evaluation," and — perhaps most damningly — a claim that "the proposals do not adequately address overall impacts on vehicle performance, emissions and safety." That last one could substantially trip Volkswagen up, because if it can't sufficiently address the cheats through a fix, it could ultimately be forced to buy back vehicles at substantial expense. (CARB says that the rejection allows for "a broader array of potential remedies.")

Update Jan. 12th 2:42PM ET: Volkswagen says that it "has had constructive discussions" with CARB more recently than the "initial recall plans" that the board is referring to in today's release. Still, there's no approved plan in place just yet. Volkswagen's full statement is below.