The California Air Resources Board has rejected a recall proposal from Volkswagen AG that was designed to fix 16,000 2009-2016 model year Volkswagen, Audi and Porsche vehicles that are equipped with its 3.0 liter diesel engines. In September of 2015, the automaker admitted that the engines include devices used to falsify emissions tests.

In its statement, CARB maintained that “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.” The regulator further said that it will require until at least December to collect enough data to determine if the fix is effective.

A spokesperson for the U.S. Environmental Protection Agency indicated that it also considered the recall plan to be inadequate.

Volkswagen responded to the decision by saying that it would continue to work with both regulatory agencies to come up with a solution.