What might a defective thermostat cause?

A defective thermostat can indeed lead to engine overheating. The thermostat plays a critical role in regulating the flow of coolant in the engine. It ensures that the engine reaches the optimal operating temperature by opening and closing at the appropriate times based on the coolant's temperature.

When the thermostat is stuck closed, coolant cannot flow through the engine and radiator as needed. This lack of coolant circulation prevents the engine from dissipating heat effectively, leading to an increase in engine temperature. If the temperature rises beyond the safe operating limits, it can cause severe damage to engine components, including the cylinder head and head gasket.

Understanding the thermostat's function highlights why its failure directly results in overheating issues. Other options pertain to symptoms or issues that can arise from different components or conditions unrelated to the thermostat's primary role in managing engine temperature.