What could cause poor heater performance?

A stuck open heater control valve can significantly impact heater performance. The heater control valve's primary function is to regulate the flow of hot coolant from the engine into the heater core. When this valve is stuck in the open position, hot coolant continuously enters the heater core, potentially leading to an inability to properly control the temperature of the air being heated. As a result, the heater may blow lukewarm air instead of hot, or it may lead to excessive heat, resulting in poor comfort levels.

In contrast, the other factors mentioned, while they also can affect heater performance, do not directly explain the failure to generate hot air in the same mechanical manner as a stuck open heater control valve. For instance, a defective temperature blend door actuator could result in improper air temperature blending but wouldn't compromise the flow of hot coolant into the heater core as directly as a stuck valve. Similarly, if the radiator coolant temperature is below the necessary threshold, it would impact overall engine performance and heating capabilities but would not involve the operational status of the heater control valve. Lastly, if the heater core return hose is much cooler than the inlet hose, it indicates problems with the heater core's heat exchange, but again does not specifically involve the control mechanics of hot coolant flow like a