Volkswagen said Thursday that federal regulators have approved its long-awaited fix for more than 326,000 vehicles affected by its diesel emissions cheating scandal. The fix, however, will cause fuel economy to decline, the company said, though it believes the changes won't hurt driving performance and reliability.

Consumer Reports will test whether the fix affects performance.

The cars, which have 2.0-liter engines and are from the 2009-2014 model years, will receive software and hardware upgrades to ensure they “function effectively in all normal driving conditions,” according to VW. They will be fully compliant with federal emissions regulations after the upgrades are installed, VW said.

The repairs include the removal of defeat device software; that software allowed the vehicle’s onboard systems to sense when the car was undergoing emissions testing and adjusted how the engine worked to reduce emissions. This resulted in vehicles that, outside of testing, polluted up to 40 times the permitted rate of nitrogen oxide during normal, everyday driving situations. 

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“VW's fix to their rigged diesel cars adds insult to injury, forcing consumers to spend more on fuel if they want to get their cars fixed,” said David Friedman, director of cars and products for Consumers Union, the policy and mobilization arm of Consumer Reports. “And if consumers refuse to make that tradeoff, more people would be left breathing dirtier air.”

“At a minimum, VW should compensate consumers for their added costs, and regulators should closely watch for additional problems,” Friedman said.

The vehicles involved in this fix make up the bulk of VW’s diesel crisis in the U.S. These “Generation One” models, from the 2009 to 2014 model years, were the oldest vehicles involved in the scandal. Fixes have already been implemented for newer models, grouped together as “Generation Two” and “Generation Three.”

Mechanical fixes include the replacement of an emission catalyst and, for 2009 model year vehicles, the replacement of the glow plug control module and diesel particulate filter. All of the modifications have been approved by both the Environmental Protection Agency, and California’s Air Resources Board.

VW said the fix for 2009 model year vehicles will require a total of six hours of work at a dealership. Newer models, dating from 2010-2014 model years, should require roughly half that amount of time to fix, VW said. Loaner vehicles should be available, VW said, for customers whose vehicles need this fix.

Owners should not notice any adverse changes, VW said, to reliability, durability, or performance (including 0-60 mph time, top speed, etc.) Still, there will be some noticeable differences, particularly regarding fuel economy, where owners may see a loss of up to 2 mpg, depending on their driving routine and habits. VW said engine noise will be louder, “specifically during moderate acceleration.”

This marks another milestone in this global controversy that led to the ouster of VW’s CEO, billions of dollars in vehicle buyback programs, and a slew of court cases.