What symptom indicates a faulty thermostat?

A constantly fluctuating engine temperature is a primary symptom of a faulty thermostat. The thermostat's primary function is to regulate the flow of coolant through the engine and the radiator, maintaining an optimal operating temperature. When the thermostat is stuck open, coolant constantly circulates through the engine, preventing it from reaching the desired operating temperature, leading to fluctuating readings on the temperature gauge. Conversely, if it is stuck closed, it can cause the engine to overheat, which may also reflect a fluctuating temperature gauge if the cooling system is suddenly able to vent excess heat.

This symptom is critical because an engine that does not operate at the correct temperature can lead to various performance issues, increased emissions, and potentially severe engine damage over time. Such fluctuations can also lead to erratic fuel injection and ignition timing, further exacerbating performance problems.

The other options relate to different issues. Excessive power output generally points to modifications or problems in the fuel delivery or air intake systems rather than thermostat issues. Poor fuel economy may be a result of a variety of factors, including fuel system inefficiencies or driving habits, and is not directly linked to thermostat performance. Rough engine starts are often associated with ignition or fuel delivery problems, rather than the thermostat's ability to regulate engine temperature