Diesel fuel gelling occurs when paraffin wax crystals in diesel fuel solidify at low temperatures, turning the fuel into a thick, gel-like substance that cannot flow through fuel lines, filters, or injectors. Gelling typically begins at the fuel's cloud point (around 32°F for standard ULSD) and becomes critical at the CFPP. Anti-gel additives like Fuel Ox Cold Charge prevent gelling down to -40°F by modifying wax crystal formation.