On October 31, 2006, Respondent operated a retail outlet that sold or offered for sale ultra low sulfur diesel (ULSD) fuel that had a sulfur content of 23 parts per million (ppm), in violation of the maximum content of 15 ppm imposed by 40 CFR Part 80, Subpart I. Upon notification of the violation, the tank was shut down and down graded to Low Sulfur Diesel fuel. Respondent did not resume sale of ULSD fuel until ULSD fuel was added to the tank and the fuel determined to have a sulfur content of 6 ppm.