A truck has insufficient air flow out of the heat and defrost ducts. The fan is operating properly. The most likely cause would be

A truck has insufficient air flow out of the heat and defrost ducts. The fan is operating properly. The most likely cause would be

• A. A clogged cab intake air filter
• B. Defective heater core
• C. Blower motor with low speed setting
• D. Disconnected duct

Answer: (A)

When the fan is moving air but the heat and defrost vents aren’t delivering much, the problem is usually a restriction in the air path before the air reaches the ducts. The cab intake air filter sits right at the entry of that path, so if it’s dirty or clogged it chokes off air entering the system and reduces the overall volume that reaches the heater and defrost vents. Replacing the filter typically restores normal airflow. A defective heater core wouldn’t mainly reduce airflow; it would affect heating performance. A blower running at a low speed setting would also cut flow, but you’d usually notice the control setting or the blower not delivering expected power. A disconnected duct would cause little or no air from some vents rather than a uniform reduction across heat and defrost outlets.

When the fan is moving air but the heat and defrost vents aren’t delivering much, the problem is usually a restriction in the air path before the air reaches the ducts. The cab intake air filter sits right at the entry of that path, so if it’s dirty or clogged it chokes off air entering the system and reduces the overall volume that reaches the heater and defrost vents. Replacing the filter typically restores normal airflow.

A defective heater core wouldn’t mainly reduce airflow; it would affect heating performance. A blower running at a low speed setting would also cut flow, but you’d usually notice the control setting or the blower not delivering expected power. A disconnected duct would cause little or no air from some vents rather than a uniform reduction across heat and defrost outlets.