Some school districts are blaming clean diesel fuel for school-bus breakdowns that left thousands of kids stranded in the extreme cold this week, The Wall Street Journal reports (via the Alliance to Save Energy).

On Monday, when temperatures dipped below zero in East Allen County, Ind., 36 of the county’s 155 school buses stopped running because the new fuel thickens in the cold. Near Pittsburgh, Hempfield area schools had the same problem. Districts in Kansas and New England reported similar occurrences.

During the refining process used to attain an ultralow-sulfur ratio, wax in diesel is affected in such a way that it can cause the fuel to turn from liquid to gel more readily in cold temperatures.

The EPA says that all diesel fuels gel in subzero temperatures and that kerosene or other additives have been added to diesel fuel for decades to prevent it from thickening.

Some school authorities said they weren’t clear that special additives or ultraclean kerosene had to be added. One school calculated that the additives cost about 37 cents a gallon.