Which condition typically indicates a malfunctioning thermostat?

A malfunctioning thermostat is commonly indicated by engine overheating. The thermostat regulates the flow of coolant to maintain an optimal engine temperature. If it fails to open or close properly, it can prevent the coolant from circulating adequately through the engine. This lack of circulation can lead to excessive heat buildup in the engine, resulting in overheating.

When the engine temperature rises beyond normal levels, it is a clear sign that the thermostat may not be functioning as intended. In contrast, a steady engine temperature would suggest that the thermostat is working correctly, while frequent coolant leaks may indicate other issues within the cooling system. Improved fuel efficiency is typically unrelated to thermostat performance and is influenced by various other factors, such as engine tuning and aerodynamics. Thus, engine overheating is a direct consequence of a malfunctioning thermostat, making it the most indicative condition in this scenario.