Automotive Service Excellence (ASE) Air Conditioner Certification Practice Exam 2025 - Free ASE Air Conditioner Practice Questions and Study Guide

A foul odor coming from the A/C vents is often associated with issues related to moisture and biological growth within the air conditioning system. Among the options provided, bacterial growth on the evaporator is a significant cause of unpleasant odors. When the evaporator is not draining properly, moisture can accumulate, creating an environment conducive to mold and bacteria growth.

This growth can produce musty or sour smells that are noticeable when the A/C is in operation. While a malfunctioning blower motor may affect air circulation, it does not directly contribute to odors; instead, it may simply reduce airflow. The clogged evaporator drain could also lead to moisture issues, but it is fundamentally the bacteria and mold growth on the evaporator that directly causes foul smells. Similarly, a dirty cabin air filter can impact airflow and air quality but typically does not result in foul odors akin to those caused by microbial growth.

Therefore, while a malfunctioning blower motor impacts performance, it does not explain the specific issue of odor, making bacterial growth on the evaporator the more accurate answer concerning foul smells from air conditioning vents.