In Feb 2018, German newspaper Bild am Sonntag reported that US authorities investigating Mercedes have discovered that its vehicles are equipped with illegal software to help them pass United States' stringent emission tests. The claimed defeat devices include a "Bit 15" mode to switch off emissions after 16 miles of driving (the length of an official U.S. emissions test), and "Slipguard" which tries to directly determine if the car is being tested based on speed and acceleration profiles. Bild am Sonntag said it found emails from Daimler engineers questioning whether those functions were legal.