If a vehicle's heating system is blowing cold air, which of the following could be a potential cause?

A vehicle's heating system blowing cold air can be attributed to several factors, and a faulty heater core is one of the primary culprits. The heater core functions similarly to a small radiator; it allows engine coolant to flow through it, transferring heat from the engine to the air that is blown into the cabin. If the heater core is clogged, leaking, or not functioning properly, it will not transfer heat effectively, resulting in cold air being emitted from the vents.

In contrast, low refrigerant levels typically affect the air conditioning system rather than the heating system. A clogged cabin filter can restrict airflow but would not necessarily cause the system to blow cold air specifically; it mainly impacts the flow of heated air. A stuck thermostat usually leads to an engine overheating issue or affects the engine's overall temperature regulation, but does not directly cause the heating system to blow cold air in the cabin if the engine is at operating temperature. Thus, the heater core's condition is crucial for proper heating functionality, validating why a faulty heater core is a viable explanation for the situation.