Symptoms Of A Clogged Heater Core And How To Fix It Old coolant is one of the leading causes of heater core blockages, thermostat failures, and general cooling system deterioration. The heater control unit is the panel you interact with to adjust temperature, fan speed, and airflow direction. The blower motor is the fan that pushes air through the heater core and out through your vents. A Clogged Cabin...