What Is a Coolant Check Valve?
A vehicle’s cooling system must be able to cope with a variety of different situations and temperature conditions, so that it can adjust as needed the flow of liquid through its plumbing system. To do so, it must consider the nature of the coolant itself, which is especially designed to provide maximum protection, but can expand and contract through the temperature range.
The cooling system is effectively sealed, so without a means of releasing pressure it would not be able to deal with the consequences (bursting) when a hot coolant expands. This is why safety mechanisms like the coolant check valve are built into the system.