Winterkorn announced in 2012 that VW would reduce vehicle CO2 emissions by 30 percent by 2015 compared to 2006 levels, but the engineers weren't able to perform the cuts. To artificially improve the results the workers raised tire pressures and mixed diesel with the motor oil to make the vehicles use less fuel. This deception occurred from 2013 through this spring, and it only came to light when an employee revealed the cheating to his bosses at the end of October, according to Reuters.
The CO2 scandal has the biggest effect in Europe where governments often link emissions and vehicle taxes. VW pledges to make things right by paying the fees for any impacted customers. When it announced the scandal, the company estimated the cost of the problem at the equivalent of $2.2 billion.
Auto industry veteran Bob Lutz recently speculated in an op-ed for Road & Track that an internal culture of intimidating workers could be blamed for the diesel emissions cheating. In that piece, he singles out former supervisory board chairman Ferdinand Piëch for pressuring employees rather than Winterkorn.