Volkswagen may ultimately be compelled to buy back cars sold in the US that violated clean air standards. The German automaker could be on the hook for nearly 500,000 models sold over several years, including the Passat sedan made in Chattanooga. The company admits it installed computer software on diesel engines that offered false exhaust emissions results. Regulators say real world driving produces emissions as much as 40 times higher than reported by the faulty computer. The company has been hit with hundreds of lawsuits since it admitted to the scheme last month. State officials say Volkswagen has insisted the massive recall will no cancel its plans to expand the Chattanooga plant for production of a new sport utility model.