Join the Car Talk Community!

Discussion Rules

Welcome to the Car Talk Community!

Want to ask a question or join the discussion? Great! Join now.

Sign In Register

Mold & mildew in the A/C

What is it with modern cars? Why does this happen so much? Both of our cars (2002 Saturn LW200 wagon & 2006 Toyota Prius) have these awful smells in the air conditioning system. When you turn it on for the first time in the spring, it puts out something that causes our eyes to water and smells like basement. I've noticed this in other people's cars and in rentals and it seems to be getting worse and worse. What do we need to get fixed and why can't automakers make this problem better? The dealerships just spray nasty chemicals down there so it masks the moldy scent but there has GOT to be a better way. Anyone???


  • N55N55
    edited May 2007
    There are a couple of things that you can do. First, make sure the rubber drain hose at the base of the firewall (on the engine side) is open. It will appear to be a closed-end rubber tube, but it has a slit in it to allow a slow drainage of moisture condensed by the a/c. Squeeze the end of the tube to make sure it is open enough to allow a slow drainage. This is often down where road grime, mud and such will "glue" it closed.

    Second, run your a/c compressor in all seasons. When you turn the defroster on, the a/c compressor should run to de-humidify air blown on the inside of the windshield. This flushes some clean condensation through the system and helps flush out old, stale water than may have accumulated in low spots. This is also good because it circulates oil through the compressor lines to help keep seals from drying out or leaking.

    Third, turn the a/c off before shutting the car down for the day. Allow the ventilation fan to run a bit as you pull onto your street to help blow moist air from the vent system.

    Modern a/c systems are properly designed, but manufacturing inconsistencies may provide "low" spots that do not drain completely. This is the source of the mold that causes the odor. If the car is still under warranty, have it checked to make sure all hoses and vents are working properly. If you have a garage, park the car with the windows down at night and allow the car to air out to help evaporate moisture trapped in the vents.

    I would use deodorants only as a last resort. They are only temporary fixes. Bleach will kill mold, but it also is temporary. You need to attach the problem, not the symptom.
  • edited May 2007
    A little careful about running the compressor during the winter, though. If it gets cold enough and there's no crankcase heater, the refrigerant can condense and mix with the oil in the compressor, which would strain the bearings and possibly cause oil slugging at startup.
  • edited May 2007
    might try turning off the A/C but keep the fan on for a few blocks before you make it to your destination to dry off the coil so it cant sit there wet collecting mold.

    Also a mix of three parts water to one part hydrogen peroxide sprayed down the front air intake grill may get rid of the smell, and kill the funk. Plus its completely orderless, and cheap
  • edited June 2007
    I've been told by the shop that repairs only Volvos that my yukky AC smell (and subsequent freon leak) is indicative of the need for a new evaporator/condensor coil (or something like that). And a significant tidbit of info re this issue is that the smell, which I describe as smelling like old, dirty socks, happens intermittently but especially when the heater is running. So $800-$900 is supposed to cure this. Will see very soon.
  • edited June 2007
    The first step is the sprays but since that did not work, you need to check to see if the condensation drain is clear and make sure the drains for the vents in front of the windshield are clear.
  • edited June 2007
    If the coil is going to be replaced, you should have them put in a coated coil, or coat a new coil with an antimicrobial coating. Others may have it but I know that Airsept does.
  • edited June 2007
    And heres another vote for turning off the AC & running the blower on high for the last 5 minutes of driving each day.

    Doing this totally eliminated the start up stink in my wifes 95 Taurus. She has continued to do this in her 2002 Sonata with the same results.

    Likewise, i've always done this in my 87 Ranger & have never had a problem with AC smells.
  • edited June 2007
    A few blocks or a few minutes won't do any good if the dew point is at or near the ambient temp:

    A lot of people complain about this problem. I am very susceptible to mold allergies so I have done some research. I think you are on the right track by looking to clean. Most people are looking for a magic spray that will fix the problem and lots of people suggest this approach -- Lysol or similar stuff sprayed into the AC system with the blower running and the windows open. This may work for a time, but it is probably only a very temporary solution. It will kill existing bacteria and fungi, but they will start to regrow as soon as the culture medium (crap) in your AC system is reinnoculated (immediately).

    First the simple solutions. Check to be sure that your AC drain is not clogged. Another simple thing to do is when you park the car be sure to leave your HVAC system in any position BUT "off", "max" or "recirc". Some owners' manuals specify this to let air circulate through the system. Try turning the temp control to full hot after parking before you shut the car off. This, in theory might circulate hot dry air from the cabin back through the evap coil by convection. Note well the "in theory". I have not tested this theory, but it seemed to help me a little. If you live where it is not too uncomfortable, turn the AC off some time before you park, it might help dry the system out. Note that running it right before parking with the heat on is not likely to help since the heater is AFTER the AC coil in any car I have looked at.

    You need to clean out your system and keep it from getting full of leaves again. Some models are easier than others to get to. Often the blower motor can be pulled out or the blower motor resistor assembly can be removed from the plenum to gain access to the AC evaporator coil. With some cars these elements are easy to get to but others are very difficult. If you can get at the coil you can spray some detergent in there. I have also used 10% bleach after the detergents. If you use the latter I would not let it sit for more than a few minutes before rinsing extensively because you could corrode aluminum parts.

    There are lots of cleaners and coatings for auto and home AC systems. Some Foaming or other cleaners rumored to be available for residential use should work. AirSept makes some coatings for Auto use. They require removal of the coil which will cost you time or money. I have not tried them.

    Some car models seem prone to this problem. It must have something to do with how well the systems dry out after shut-off and how much crud collects in the "coil" (Now, I believe, many manufacturers use flat-plate heat exchangers.) I hope that the answer to the problem will be effective filters to keep the food away from the microbes that like to grow in moist, dark places.

    I have spent a lot of time on this problem and I've had some generous help from people on web fora. Getting information about the auto manufacturers' equipment was not easy. It seems that not many people out in the trenches (auto techs), even the factory-trained ones, know about them.

    Despite the attempts at secrecy, I have found out how some of the manufacturers remedies work. Ford has a kit and a TSB (02-11-7 ,JUN 02, A/C System - Musty/Mildew Odors) . There is an "afterblow" or "purge" module and two wire kits to make it fit their various models. The price would be about $200 total depending on what wiring harness is needed. It runs the blower on high for a couple of minutes some time after the AC and car are shut off. The delay is important because no cold surface is going to dry with hot, moist air blowing over it. It also prevents operation at convenience store stops. I have also read recently that Ford no longer offers this stuff because of run-down battery problems. I don't recall where I read this and I have not checked the facts.

    Saturn has a kit for about $125 (delayed blower motor controller: 21031158). It seemed to be adaptable to any car. I was told that it runs the blower for 5 minutes after the car is turned off for 50 minutes to allow the coil to warm up.

    Airsept probably still has a web site. They also make an afterblow module that is universal. The cost was about $300 to AC techs, I believe. It is costly, but it should work better than the others. Look a the site for the features. It starts a pulse cycle after giving some time for evap to warm. The pulse operation is supposed to squeeze the most out of your battery's electrons by allowing the evap box warm up a little between the drying (cooling) pulses. It also has battery protection. It sounds good to me.

    Airsept also makes cleaners and coatings to prevent microbe growth. These should also work. For most, however, you need to take the system apart to coat them. Note that there are foaming cleaners available for home HVAC. They should also work. Note that DSS (Dirty Socks Syndrome) is an issue with home units as well.

    I think the problem might, in some cases, be solved with copper coils since copper would inhibit microbe growth. On the other hand, when all cars have cabin air filters the microbes will no longer have a food source for the microbes.

    I live in a very hot/humid place. I pulled the blower motor from my car, cleaned and disinfected the evap. Fortunately one side is is readily accessible in my car. The evap looked sparkling clean, before I started so the microbe growth might be somewhere else in the plenum. I decided that I could make my own afterblow module with a little time and a soldering iron. I saved a lot of money and got great performance.

    I went through several designs and decided that, in my climate, on most days when the humidity is high, I would need an auxiliary battery to dry the evaporator out with the HVAC blower in combination with systems like the commercial ones. I decided to reverse the afterblow. This blows cool, dry cabin air past the heater core and the evap. I made it with a small, plastic squirrel-cage blower, an inexpensive "multimode" timer kit and a couple of automotive relays. I plumbed the blower in to the passenger foot well heater outlet (not as easy as it sounds). I have a seasonal change-over of removing this and taking the duct tape off the other 3 heater outlets.

    During AC season (10-12 month) I have to remember to put the system in the "floor" mode and turn the heater to high as I park. I have the timer triggered on the AC control so it activates if the AC has been on and runs the little blower for 2 h. I have a reset button and an on/off switch stuck behind the lighter in the ashtray. All the equipment is tucked up behind the dash. I have also added a little $20 pneumatic valve that opens the dash vent ducts when the blower stops to allow better circulation in case there is any residual moisture in there.

    My solution is obviously not a universal solution, but it works for me and was inexpensive. My total costs were higher because I had a lot of false starts, but to repeat it would probably only cost about $30 excluding the valve. I did spend a lot of time on it.

    The "Multimode Timer" kit is available at several mail/wwweb places. A couple:

    Manual at:
This discussion has been closed.