Q: Why Does the Air From My Air Conditioner Smell Bad?

Algae or Mold Growth

The source of the smell from your vehicle’s heating and air conditioning system is the result of mold and or algae growth around the evaporator and the evaporator drain. When the growth of mold and algae occurs and air is pushed through the evaporator case and out your A/C vents, you will smell the flora growing inside of the evaporator case.

This growth occurs because the evaporators cooling effect creates condensation on its surface. You can see the results of a functioning A/C system whenever you see clear water dripping from under your car. This is a normal byproduct created from a rapid cooling of the air surrounding the evaporator. When the water that is created sits in a closed container (your evaporator case) algae and mold will grow in the same manner as it would occur in your shower if left uncleaned. Of course the evaporator case is much harder to access for thorough cleaning.

In most conditions the water created by the evaporator will evaporate after the system is turned off. There are a few conditions that will hold moisture in the evaporator case. This is a common problem on all vehicles and occurs more often in climates in climates that are high in humidity or rain. Clogging would be the other condition that will hold water in the evaporator case.

Evaporator Drain Clogging

Aside from humid climates, clogging of the evaporator drain will cause water to puddle in the evaporator case allowing algae and or mold to grow. Evaporator drains can become clogged with debris from mice and or the buildup of algae. In either case, it can be challenging to thoroughly remove the clog and sometimes requires removing and disassembling the evaporator case. In some vehicles it is possible for debris from trees or other objects that maybe above a parked car to enter the evaporator case through the air intake for the heater and air conditioning system. This is less of a problem on modern vehicles, nevertheless, I would not advise you to park under trees or other objects that may shed debris.

If debris is lodged in the evaporator drain, it must be removed or the smell will persist. Clogged evaporator drains can also leak water out of the case onto the floor creating a wet carpet. Many a people have confused this condition for a leaking heater core. For this reason it is vitally important to determine if the wet carpet is water from condensation or antifreeze. Antifreeze will have an oily texture with a sweet smell and taste. Some technicians will actually taste it. I for one do not and wouldn’t recommend doing this.

Cleaning an evaporator drain often requires patience and some creativity. Many times it is simple, but on those occasions that it is not, it can be very frustrating. There is usually a tube at the exit of the evaporator drain that directs the water out and onto the ground. Usually a screw driver, wire, or a air nozzle is sufficient to clear this tube. After clearing the drain, you may still have smell coming from your vents for a short while, but the smell should subside fairly quickly.

For more problematic smells that will occur in humid climates, it will be necessary to inject lysol or some other mold and algae killer into the evaporator case. Again, this is often a creative process that can be frustrating. The best method to prevent foul smells from your vents would be preventative.

Cleaning Out Algae and or Mold

Once algae and or mold has established itself in the condenser/heater case, it must be cleaned out before you will be able to keep the smell away. Flora of this sort can dry out and but still live on once more moisture is introduced. Prevention is only possible if there hasn’t been a significant amount of growth established. Small amounts can be killed and will be flushed out with preventative techniques, but if a sufficient amount is already present, it will need to be thoroughly cleaned.

If this is the case, it usually takes multiple treatments of lysol or other disinfectant to kill off any growth. There are a number of products on the market for this purpose, but the problem isn’t the choice of what product to use. The problem is getting the disinfectant inside the case to the affected area. Modern evaporator/heater cases are complex systems that require extensive disassembly to access the evaporator compartment. Unfortunately the best way to clean the evaporator case is by disassembly and manually removing all algae and or mold.

If disassembly of the condenser case isn’t an option, the only thing that can be done is to do the best you can to get a disinfectant onto the algae and or mold. One method in doing this is to spray lysol into the intake of the vent system. The intake for your A/C system is located in the cowl area of your car. This is where your wiper blades are attached. Do this with the blower motor on so it will suck in the disinfectant into the interior vent system. The drawback from this technique is the smell of the product you spray into the intake vent will be sprayed into the entire passenger compartment. This will most likely need to be done multiple times until the algae and or mold has died and been flushed out the evaporator drain, so choose your disinfectant wisely. You will likely have to live with it for awhile.

Foul Smell Prevention

Preventing the growth of flora is the key here. If you already have a smell coming from your vents, it maybe necessary to kill and or remove the algae or mold before the following method will be effective. Barring a clogged evaporator drain, there are a few things you can do to prevent mold and or algae growth in your evaporator case. Sometimes these methods are easier said than done, but if smell from your vents is problematic, there are a few things you can do.

The key to preventing the buildup of mold or algae is the drying of the evaporator case. This can be done by turning the A/C off a few minutes before arriving at your destination. You still want the blower fan to be on in order to move any moisture out of the evaporator case. This doesn’t have to be done every time you drive your vehicle, but pays off if done occasionally. In some cases it may be helpful to turn the temperature up to further aid the drying of the evaporator case. Of course this could be highly uncomfortable in some situations.

My personal method is usually done in the winter months. I begin by turning the heat all the way up with the A/C off. Once the inside of the cabin becomes near unbearably hot, I open all the windows and adjust the fan speed to maintain a comfortable level while I drive with the windows down. This method enables me to lengthen the time I can run hot air through the heater/evaporator case and thoroughly dry out any moisture that might be in there.