The most common risk associated with running a car engine with low coolant is of overheating, which can rapidly cause breakdowns in any engine part. In the winter, coolant also acts as antifreeze. Too low a concentration of the active ingredient, ethylene glycol, allows the water in the radiator to freeze in low temperatures.
In an engine that overheats, buildups of pressure can cause tubes and gaskets to burst and metals to crack. Overheating, particularly in the summer, can abruptly render a car non-functional and subject to costly repairs. In the winter, freezing can prevent the cooling system from flowing at all, leading to the same outcome.
A car's coolant system, in addition to motor oil, compensates for the fact that combustion engines naturally generate more heat than the tolerances of the materials from which they are made. In this system, a mixture of chemicals and water is pumped through tubes throughout the engine, absorbing heat in a circuit that begins and ends within the radiator. There the mixture is cooled by air drawn in by the car’s forward motion. Because the same coolant is then cycled back throughout the engine, the volume of coolant directly determines the amount of heat that can be absorbed per cycle.