Good soaking in diesel. Like most fuels, diesel is a mix of hydrocarbons, and the components have different freezing points. For Number 2 diesel, as the ambient temperatures drop toward 0°C (32 F), it begins to cloud, due to the paraffin in the fuel solidifying. As the temperatures drop below 0°C, the molecules combine into solids, large enough to be stopped by the filter. This is known as the gel point, and generally occurs about -9.5 degrees C (15 degrees F ) below the cloud point. This wax then forms a coating on the filter which results in a loss of engine power. The same thing happens on starting an engine when the temperature is below freezing. The filter becomes almost instantly coated with wax - usually, enough fuel gets through to allow the engine to idle, but not attain operating RPM. There are two common ways to overcome this: one is a diesel additive, the other is a fuel heater. In Alaska and other colder climates, lorries are running regularly at -46°C (-51 F) or lower, so as you see, it depends on additives and heating. But to freeze - as in turning solid - you would need laboratory conditions; nature cannot go cold enough to freeze to a low enough temperature.