The California Air Resources Board told Volkswagen on Friday that it would take three more weeks to review the automaker’s proposed fix for its 2-liter diesel engines after the automaker added “significant” information to its plan, according to a letter sent by regulators.

The letter indicated that Volkswagen had submitted “additional significant information” to the board Dec. 14-16 regarding its proposed fixes for its illegally polluting cars and that the board would take until Jan. 14 to review that additional data. On Nov. 20, Volkswagen submitted its plan to CARB to fix more than 482,000 cars in the U.S., which could have been approved as early as Dec. 22.

It’s unclear from the letter what the additional information from Volkswagen may be. The automaker didn’t immediately comment on the letter.

According to the letter, Volkswagen submitted a request Dec. 15 to extend submission of its recall plan to the board. CARB officials said they would take until Jan. 14 to review that plan as well.

This month, German transportation officials approved a plan by Volkswagen to fix its European diesel cars. That plan, which affected 1.2-, 1.6- and 2-liter diesel cars, will begin in January. All of those cars will receive a software update, while some will need an additional mesh pipe added to the cars’ intakes.

Volkswagen officials said fixes for North American cars would be significantly different, although some of the cars would only need a software update. The vast majority of its diesel cars in the U.S. will need additional hardware along with software to comply with emissions standards.

CARB’s letter to Volkswagen only addressed the 2-liter diesel engines. Volkswagen has until February to detail its fix for 85,000 3-liter diesel engines that cheated emissions tests.