Final answer:
A malfunctioning engine thermostat is indeed a cause of poor heater performance as it can prevent the engine from reaching the optimal temperature for efficient heating. The statement that a malfunctioning engine thermostat can be the cause of poor heater performance is true.
Step-by-step explanation:
The thermostat in a vehicle regulates the engine's temperature by controlling the flow of coolant to the radiator. If it malfunctions, the engine may not reach its optimal operating temperature, which is necessary for efficient heater performance. A cold engine will result in the heater blowing cool air, and consequently, it may feel like the heater is not performing well. Additionally, this can lead to decreased fuel efficiency and increased wear on the engine, as the vehicle may run colder than intended.