Minnesota has relaxed its biofuels mandate by temporarily suspending a requirement for No. 1 diesel fuel to be blended with 5 percent of biodiesel for the remaining duration of the winter, said state officials.

The decision, which took effect Friday (1/15), was arrived at after filtering system problems were discovered in trucks in extremely cold parts of the state, said Phil Smith, a spokesman at the state Department of Commerce, which enforces the biofuels mandate.

He said the exact cause of the problem is not yet known and tests are ongoing at various labs to determine why a 5 percent biodiesel blend would have adverse effects on the truck filters.

Kelly Strebig, a researcher at the University of Minnesota’s Center for Diesel Research, said it has also been observed that the biodiesel blended at 5 percent or more would drop to the bottom of the fuel tank in subzero temperatures.

The suspension of the mandate only affects No. 1 diesel, because there has been no biodiesel blend problem with No. 2 diesel. In fact, there have been no reports of problems with No. 1 diesel blended with a 2 percent biodiesel.

“I’m at a loss as to why this happens only in a 5 percent biodiesel blend,” said Strebig.

No. 1 diesel is used mostly in the construction industry.