The California Air Resources Board rejected Tuesday Volkswagen's recall plan for diesel cars equipped with emissions cheat devices, and the federal Environmental Protection Agency agreed the plan was not acceptable.
CARB said VW's proposals for its 2.0 liter diesels, submitted in November, "lack enough information for a technical evaluation" and "do not adequately address overall impacts on vehicle performance, emissions and safety."
The EPA, itself awaiting VW's formal proposals, backed the California regulator.
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The statements came a day before Volkswagen's chief executive, Matthias Mueller, is scheduled to meet in Washington with EPA officials to discuss how the company plans to deal with 600,000 of its cars in the US equipped with illegal "cheat devices" that allowed them to cover up excessive emissions.
The issue, which extends to 11 million VW, Audi and Porsche 2.0 and 3.0 liter diesel cars worldwide, has severely damaged Volkswagen's reputation and spawned a host of investigations in several countries.
In the United States alone the company is facing potentially tens of billions of dollars in fines over the scandal.