What could cause foul odors coming from the AC system?

Foul odors from the AC system are often the result of mold or mildew buildup within the components, most notably in the evaporator core and any moisture-prone areas of the system. When the air conditioning is in use, humidity can condense and collect in these areas, providing an ideal environment for mold and mildew to thrive. This can lead to unpleasant smells being circulated through the vehicle’s cabin when the AC is engaged.

While low refrigerant levels, failed compressors, and dirty air filters can cause various operational issues with the AC system, they are not typically associated with foul odors. Low refrigerant may lead to a reduction in cooling efficiency, a failed compressor might result in system failure altogether, and dirty air filters can restrict airflow but do not tend to produce the distinct musty smells linked to biological growth. Thus, mold or mildew buildup is the primary factor responsible for the foul odors in the AC system.