The Environmental Protection Agency (EPA) and Volkswagen are continuing talks on how to repair VW, Audi, and Porsche 3.0L diesel engines for cheating on emissions tests, after California regulators reject VW’s most recent recall proposal. In a letter to Volkswagen, CARB listed the failures of the proposed solution, saying that the company failed to provide a full description of the defeat devices, the impact the fix would have on vehicle performance and emissions, or even a description of the fix that would allow CARB to evaluate its feasibility.

The announcement is a setback for the automaker who believed that the 3.0-liter TDI models sold by Porsche, Audi and Volkswagen would only require a software update. If the manufacturer is unable to come up with a satisfactory fix, they could be forced into buying back almost 85,000 vehicles. VW also continues to face lawsuits by investors and dealerships in the U.S., as well as from consumer in Germany.