An auxiliary tank (expansion tank) is one part of a complex overall car cooling system that’s designed to keep the engine operating at exactly the right temperature.
In the sealed cooling system that’s present on most cars, it is partially filled with air.
The air in the expansion tank provides a ‘cushioning’ effect that allows the heated coolant to expand without causing the cooling system to fail.
This is to prevent the system from becoming over-pressurised as the coolant heats up and expands.
To help avoid and prevent premature failures, your expansion tank should be checked annually.