How to get HVAC airflow on heat setting?

ID Status Date Year Make Model Transmission Type A/C Controls Public/Private
#14102 Closed 2003 Cheverolet Impala LS public

HVAC airflow problem, cold blows fine, no SCFM on heat setting, blower motor still running. Selector switch for vent, defrost and floor works fine and plenty of airflow on cold / A/C and blower speed selector switch works fine. When dual zone sliders are moved to heat, airflow drops to almost/basically nothing. It seems that there is no airflow through the heater core or a door is not opening. There is heat, just no airflow and it takes some time to get the cabin warmed up on a cold/freezing day. Any thoughts would be appreciated.


Sorry I have to ask. What does SCFM mean?

I am gathering that there is no air flow out of the vents when the slide controls are moved to heat. Is that at full heat or just anywhere off of full cold? If it is only doing it at or near full heat my first suspicion would be something blocking the fresh air intake (restricted cabin air filter).

SCFM is Square Cubic Feet per Minute, an airflow measurement, also seen as just CFM. Your shop air compressor should have this rating at a certain PSI.

On full cold there is full airflow. As the temp slider is moved up toward the hot side, airflow diminishes accordingly no mater which outlet is selected. It is not the cabin air filter / fresh air. I have replaced the filter and glued the weatherstrip above it in place. The problem was around before I did this. Also, the problem is the same whether recycle/cabin air or fresh air is selected.

Is there a “door” that will block the Heater Core air flow?


According to GM service information there are two doors but the way in which they function would indicate that there is a physical obstruction on the heater core itself. Please see the attachment.

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