I had this on a previous car and it was condensation/ moisture in the lines/vent...
when it first came on it was the worst.

That would explain the smell mitigating when you turn on the ac... as the ac serves as a dehumidifier....

It can be fixed, just isolating where it is, and finding out how the moisture is getting in there and getting musty/moldy.... good luck

 

Thanks for sharing your experience, JohnBoy Too.
It sounds similar to the problem you had.

Did you actually have moisture leaking into your vents?
I live on the "wet" (west) coast, so there is a lot of rain that falls. Could it be when the fan vent is on, that "wet" air is being pulled through?

 

I would challenge your definition of wet - it's all relative as I worked in Louisiana and the wet was from above and below !

Not sure of the source as I brought it to the dealer and they fixed it -
IIRC, when I asked, they either cleared a drain tube, or reran it as it was kinked -
think the high humidity there made it worse than it would have been...

good luck and if you park outside, have also heard that it could be animals making a home in the nice warm confines of your engine bay....

 

you have to buy the cleaner, and do the full system clean. It's very DIYable...and the cleaner only costs about $20.

Or you could have the dealership do it for about $180.

 

I went to my Audi dealer and they confirmed this bad smell from the air vents is completely normal if you don't use your A/C once in awhile during the winter months.

They have a treatment for 59.95$ that cleans out all build-up bacteria in the vents.

 

Thanks for all your suggestions. I think the smell is from the residual moisture after the a/c is turned off.

My mechanic did the a/c cleaning, but eventually the smell does come back. He said that some clients turn on the "heat" for a couple of minutes before they turn off the ignition to "dry out" the system.

 

I have the same problem.... I don't care but now that my wife is Prego she doesn't even drive the truck. apparently her sense of smell has heightened

 

I also have a 2002 Jetta and it never had this smell from the vents when the a/c is off.

 

my Q5 has the exact same problem....once I turn off the a/c, the smell will blow out immediately and my wife always ask if I just farted....
PS. she sits at the back seat along with my 16 months old son, so most of the time she doesn't see me switch the a/c off.

 

I'm sure glad I don't have this problem.....my AC just died.