I recently left my car (Honda Accord) in the parking lot of my apartment complex for a little over a month while I was out of town. It was working fine when I left.

Now that I've come back, I've noticed something visible coming out of one of AC vents (only when the AC is on). Sort of like a light smokey look-- but it's not actually smoke. The other AC vents act normally. It has a slight smell, but I can't identify it.

Otherwise the car looks, sounds, and feels fine to drive. Does anyone know what might be wrong with the vent and if there's a simple thing I can check to fix?

I currently live in Houston, TX so it was very hot and humid for the month that I was gone-- not sure if that could have caused anything.

  • 2
    It might just be the cold air condensing the moisture in the air creating fog.
    – Tester101
    Commented Jul 20, 2011 at 16:27
  • I don't think this is it... It's only out of one vent (always the same), and fog doesn't really have a smell in my experience.
    – Jeff
    Commented Jul 20, 2011 at 19:42
  • The fog may only come out of one vent, say for example if it's the first vent in the group so it possibly gets more flow. Or also depending on the lighting in the vehicle, may only be visible from that vent. I just experienced this this morning on my way to work, I had the AC blasting and fog was coming out from a single vent.
    – Tester101
    Commented Jul 20, 2011 at 20:38

1 Answer 1


That sounds like condensation from moisture that's already in the system. It's not harmful but that slight smell could be the beginnings of mildew.

You might want to dry out the system. The best way that I've heard of is to run the air through the recirculator with the heater on, trying the defrost, floor and vent settings. If you run hot dry air through the system for a while, it'll dry out quite a bit.

Admittedly, in Houston this will be like sitting in the ninth circle of Hell....

Afterwards, you should notice that you aren't getting that fog from the vent (or at least quite a bit less).

  • It's like a sauna within a sauna... Anyway, thanks I'll give this a try!
    – Jeff
    Commented Jul 20, 2011 at 19:45
  • Also check out mechanics.stackexchange.com/q/116/37 for a bit more info on this.
    – Rory Alsop
    Commented Jul 20, 2011 at 20:17
  • 2
    Nobody says you have to sit in the car when you run the heater, or that you have to keep the doors/windows closed. Turn on the car, blast the heater, then run back inside to the air conditioned house. The moisture will not be removed any faster just because you are watching.
    – Tester101
    Commented Jul 20, 2011 at 20:42
  • 1
    @Tester101, good point on sitting. However, there's a good chance that running the recirculation with the windows closed will lead to dryer conditions in the air channel. The driver has to make the call, of course.
    – Bob Cross
    Commented Jul 21, 2011 at 0:15

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