Volkswagen's proposed fix for roughly 85,000 illegally polluting 3.0-liter diesel engines isn't good enough, a California Air Resources Board letter said Wednesday.

The air quality regulators rejected VW's plan and said the proposals were "incomplete, substantially deficient, and fall far short of meeting the legal requirements to return these vehicles to the claimed certified configuration," according to the letter.

The rejection is a setback for VW, which claimed last month in court that it was nearing a fix for the 3.0-liter diesel engines that were equipped in several Audi models as well as Porsche and VW SUVs.

"The company believes that we can fix the 3.0-liter to the standards to which those cars were originally certified," VW lawyer Robert Giuffra said, according to Reuters.

The CARB's letter to VW not only casts a doubt on the company's ability to fix the cars, but also slammed the automaker for not disclosing more information about its "defeat device" that regulators alleged skirted emissions laws by cheating through testing. The letter also said the automaker wasn't fixing the cars fast enough; data for the fix wouldn't be complete until December.

2015 Audi Q7 TDI

2015 Audi Q7 TDI

A spokeswoman for Volkswagen said the rejection had been anticipated by the company.

"We understand that today's announcement from the California Air Resources Board (CARB) is a procedural step under California state law and relates to recall plans for vehicles with V-6 3.0L TDI engines that were submitted previously.  We continue to work closely with the U.S. Environmental Protection Agency (EPA) and CARB to try to secure approval of a technical resolution for our 3.0L TDI vehicles as quickly as possible,” VW spokeswoman Jeannine Ginivan said in a statement.

According to Car and Driver, roughly 16,000 of the affected 85,000 cars and SUVs were sold in California, whose state air quality officials are helping the EPA.

Last month, Volkswagen announced an unprecedented $15 billion plan to buy back cheating diesel cars, including penalties for skirting emissions laws, though that didn't include these larger 3.0-liter models. Experts estimated that if the automaker were forced to buy back luxury models equipped with the bigger 3.0-liter models, it would add billions more to the overall cost.

The vehicles affected are the Volkswagen Touareg TDI, the Porsche Cayenne Diesel, and five Audi models: The A6, A8, and A8L TDI sedans; the A7 TDI hatchback; and the Q5 and Q7 sport utility vehicle.