revision preference src: Which starting in June 2006, petroleum refiners were requires by the EPA to begin producing ultra-low sulfur diesel (ULSD) fuel. Which has 97 % less sulfur than the previous low sulfur diesel fuel. <SEP> tgt: Starting in June 2006, petroleum refiners were required by the EPA to begin producing ultra-low sulfur diesel (ULSD) fuel, which has 97 % less sulfur than the previous low sulfur diesel fuel.