Faulty thermostat: The thermostat is responsible for regulating the flow of coolant to the heater core, and if it's faulty, it may not be allowing enough coolant to flow through, resulting in cold air being blown through the vents.
Blend door actuator: The blend door actuator is responsible for opening and closing the blend door, which controls the mix of hot and cold air that flows through the vents. If the blend door actuator is faulty, it may not be able to properly open or close the blend door, resulting in either cold or hot air being blown through the vents.
Faulty heater control valve: The heater control valve is responsible for regulating the flow of coolant to the heater core, and if it's faulty, it may not be allowing enough coolant to flow through, resulting in cold air being blown through the vents.
