Which statement about thermostats is TRUE?

The statement that the thermostat opens at its marked temperature is indeed accurate. The thermostat is a crucial component of the engine cooling system, designed to regulate the flow of coolant from the engine to the radiator. It operates based on temperature; when the engine reaches the temperature specified by the thermostat's rating, it opens to allow coolant to flow through the radiator, enabling the engine to maintain an optimal operating temperature.

The operating principle behind the thermostat is that it ensures the engine warms up to the correct temperature efficiently and then allows for proper cooling once that temperature is achieved. This process prevents the engine from overheating while also allowing it to reach optimal performance levels quickly.

Understanding this function highlights its importance in maintaining engine temperature. The other statements lack accuracy regarding the general characteristics and operational protocols of a thermostat. For instance, while the longevity of a thermostat's service life can vary, a two-year replacement is not a standard recommendation, and thermostats do not adjust their opening based on engine RPM—they depend solely on temperature readings. Additionally, stating that the thermostat has no effect on engine cooling undermines its critical role in managing coolant flow to regulate temperature.