Seems like this is a common problem:

What's That Smell??
by Vincent Ciulla
for About.com
In the last few weeks I have received a few letters asking me how to get rid of the smell coming from the air conditioning vents. This has become an increasingly common complaint. It seems that this type of complaint is coming from owners of late model cars and almost always cars with R-134 systems.

This is not a new problem; it's been around ever since there have been air conditioners in cars. But if this is an age-old problem, why is it becoming more common? Before we can figure out how to get rid of that smell, we have to understand what's causing it. The origin of the smell is caused by fungus, bacteria and other microbes growing inside the evaporator core. The moisture-laden environment is very conducive to the growth of these organisms.

As automakers downsize components to save space and weight, this problem has been increased. Because the automakers made the evaporator smaller, the added more fins and packed them closer together to increase the efficiency of the evaporator. While this has made the evaporator more efficient, it has also made it more prone to trap moisture that contributes to the growth of these organisms.


source: http://autorepair.about.com/cs/heati.../aa071601a.htm

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