Diesel Fuel Supplement +Cetane Boost is a winterizer/antigel that is used in the cold winter months to prevent fuel gelling and keep fuel-filters from plugging with ice and wax. When temperatures drop, paraffin (wax) in ULSD fuel will gel stopping fuel from flowing through the engine and water in the fuel can freeze on the facings of fuel-filters, blocking fuel flow. It provides trouble-free winter operation for diesel fuel. Diesel Fuel Supplement +Cetane Boost is intended for use only during cold winter months when temperatures drop below +30F. Use Diesel Kleen +Cetane Boost (in the silver bottle) for peak performance in non-winter months. If your vehicle will not start or gain power in cold temperatures, use Diesel 911 (in the red bottle).