My girlfriends 2007 S40 has a strange smell when you run the AC. Its not noticeable at all when the heat is on, just the AC (from the regular vents and in defrost mode). At first I was thinking the cabin filter but the fact that it doesn't smell when the heat is on kinda threw me off. Any suggestions?

 

I would check your cabin filter anyways just to make sure..you might have a mild growth of fungus of mold in the ac system. One thing you could do is spray lysol on the underside intake and let it circulate thru the system. That will hopefully take care of that...

 

First we'll need to know what kind of smell is it.

Is it like a musty type smell?
Is it a chemical smell?
Is it a rotting smell?

 

Wouldn't rotting and musty go in the same category? Usually means bacteria brewing in the HVAC system OR Filter. If it was chemical then I would start at the heater core however, he did say the smell isn't there when the heater is on. Heat will normally dissipate bacterial smell........

 

The point is that it could be a bunch of different things all the way from dust and bacteria to leaking fluids.

We can't discount the possibility that a small rodent got into the system and...perished...I've heard it on CarTalk before.

 

Its kinda musty but a little worse. We try to avoid using the AC so Im not sure how to describe it. Next time I'm in the car Ill run the defrosters and get a better description. Its sort of like a stale funk.