California air regulators have rejected Volkswagen AG's proposed remedy for its 3.0L clean-diesel engines that violate emissions laws as "substantially deficient," according to a July 13 letter to the automaker.

The California Air Resource Board said Volkswagen's submissions to address the violation are incomplete and "fall far short of meeting the legal requirements" to bring the vehicles into compliance.

Vehicles fitted with the 3.0L diesel engines include the Audi A6, A7, A8, Q5, and Q7, as well as the Volkswagen Touareg and Porsche Cayenne.

The board listed 10 elements of the plan that need improvement, including a sufficient description of the remedial procedure, parts availability, and description of how the fix affects fuel economy, drivability, performance, safety, and emissions levels.

The letter was sent to David Detweiler, Volkswagen's executive vice president and general counsel, from regulators representing the Air Resource Board and U.S. Environmental Protection Agency, including the board's Todd Sax, enforcement chief, and Byron Bunker, the EPA's compliance director. Read the full letter here.

Regulators sent a separate letter to Porsche stating that the company submitted an incomplete draft recall plan on Feb. 2.