a method for checking the accuracy of an oxygen sensor based determination of the alcohol content of the fuel of a flexible fueled vehicle . following the addition of fuel to the flexible fueled vehicle two alcohol concentration possibilities are calculated . the two alcohol concentration possibilities are then used to calculate thresholds for the determining if the oxygen sensor based determination falls within the thresholds . if the oxygen sensor based determination does not fall within the thresholds then action is taken to counteract an inaccurate determination of alcohol concentration .