Following the Dieselgate saga when Audi claimed it had an easy, software fix for the emissions tests violation, the California Air Resources Board has rejected the Volkswagen Group’s recall solution to the matter, saying the automaker’s fix is “incomplete and deficient in a number of areas.”

In an Automotive News report, the regulator said it will not have enough data before December to determine whether or not the fix for the 3.0 litre engine will work for all of the Group’s vehicles. If no solution arises, the car manufacturer may have to buy back the vehicles, which could add billions to the cost of its buy-backs.

In November 2015, CARB and EPA (Environmental Protection Agency) found that the the 3.0 TDI engine contained undisclosed auxiliary emissions control devices. At least 85,000 vehicles with the VW Group’s 3.0 diesel engine were admitted to have defeat devices, involving Audi, Porsche and Volkswagen models from the 2009 until 2016 model years.

The 3.0 diesel engine makes up a relatively small portion of the Dieselgate saga. The EA189 2.0 litre diesel engine accounted for approximately 480,000 vehicles affected by the emissions control defeat devices, eventually leading to VW allocating RM3.9 billion to compensate US owners of affected cars. Customers in the automaker’s home market of Europe, however, had no such luck.