What Causes the Car AC to Smell Bad? Moisture Your vehicle’s AC system does not actually create cold air. The AC system takes heat and moisture out of the air that is outside of your car, which in turn, makes it feel much cooler than the air around you. Hot air flowing into the unit can create moisture pockets that surround the condensing unit in your AC system. This may create small pools of water that are drained outside of the car. However, if moisture becomes stuck or for whatever reason cannot find its way out of the AC unit, by drainage or evaporation, the water will pool inside the unit and around the coils creating moisture inside of the AC unit. Mold AC systems collect moisture which creates mold. The air that passes through the evaporator and the units’ mold build-up will carry the foul smell inside of your car through your dash vents. Mold is the most common culprit for causing smells to come out of your cars’ AC. As a vehicle gets older, bacteria, mold, and other unwanted micro-organisms can start to grow. If the AC unit is not often used, or used much too often, this can also increase the growth of these microbes inside your car’s AC system and ventilation system as a whole. Odor-causing mold loves to reside in your cars’ AC unit because it is always moist and dark – ingredients for a happy home for mold. How to get rid of the smell: Here are some ways you can do so… Turn off the AC 2-5 minutes before you shut off your vehicle but leave the fan on. This will allow the entire AC unit to dry much more thoroughly every time you use it. Clean or replace your AC filters. Dirty air filters reduce airflow through your AC and help moisture remain inside the unit. Be sure to replace the AC filters as often as recommended by the manufacturer of your vehicle. Spray an antiseptic into the exterior air vents (where your car pulls air from outside into the car from). Be sure your AC is set to the setting that pulls air from outside in, so that the spray is pulled into the ventilation system and can directly work at killing of mold and bacteria.