Why is there a strange odor when the car's external air circulation is on?
3 Answers
If there is an odor when the car's external air circulation is on, a short-term smell might be due to external air odors, while a persistent odor could be caused by mold or foreign objects in the air intake ducts or a dirty car air filter. Car internal and external air circulation: 1. Internal circulation: In internal circulation mode, the airflow passage between the inside and outside of the car is closed. Without the fan, there is no airflow circulation. When the fan is on, the airflow drawn in comes only from inside the car, creating an internal airflow cycle. Internal circulation mainly prevents external dust and harmful gases from entering the car promptly and effectively. Another function is to maintain temperature. 2. External circulation: External circulation mode uses the fan to draw outside air into the car. This means the air passages between the outside and inside of the car are open, and the air blown by the fan comes from outside. Even if the fan is off, airflow is still drawn into the car while driving to replenish fresh air inside.
I've been driving for over twenty years and have encountered the issue of strange odors when using the exterior air circulation several times, like musty or burnt smells, which is really unpleasant. At first, I thought it was due to poor outside air quality, like truck exhaust or heavy dust, but the problem mostly lies in the air conditioning system. The cabin air filter gets dirty over time, sucking in external pollutants, and the evaporator tends to accumulate moisture and mold, breeding bacteria that cause weird smells. My suggestion is to handle it simply: replace the filter yourself every few months, use some deodorizing spray to clean the ducts, and if that doesn’t work, take it to a repair shop for a thorough evaporator cleaning. Ignoring the issue long-term not only affects your driving mood but can also clog the AC vents, making the car less fuel-efficient. This minor maintenance isn’t expensive and can be sorted out with a little time.
From a technical perspective, the causes of odors when using the external air circulation mode are quite common and generally fall into three main issues with the air conditioning system. The primary culprit is a moldy or waterlogged evaporator, especially during humid rainy days when bacterial growth produces a rotten egg-like smell. An overly old filter can also accumulate dust and microorganisms, introducing contaminants into the cabin when external air circulates. Additionally, loose or cracked pipe connections may allow external pollutants to seep in. The solutions are straightforward: first, inspect and replace the filter if dirty; then, blast high airflow to disperse mold buildup. If the problem persists, a technician should use specialized cleaning equipment to flush the system. Regular maintenance can prevent these issues—an annual service is hardly any trouble.
