Diesel fuel is prone to gelling in cold conditions, which can clog fuel filters and prevent the fuel from flowing properly. Generally, temperatures below 32°F (0°C) can start to cause problems for diesel fuel, and temperatures below 15°F (-9°C) can cause it to gel and become almost solid. However, the exact temperature at which diesel fuel will begin to gel can depend on factors such as the type of fuel, the age and condition of the fuel, and the specific additives used in the fuel. It is recommended to use a winter blend diesel fuel or use anti-gelling additives when operating in cold temperatures.