Towards the end of last summer my 2007 Toyota Yaris developed a musty smell in the cabin whenever I ran the A/C. This is a common problem in many cars but the Yaris seems particularly susceptible, probably because it lacks a mesh screen behind the air intake vents below the windshield so debris can fall in and block the HVAC condensate drain.
Over the winter everything dried out and the smell went away but on the first nice day this spring, when there was a little humidity in the air, the smell came back.
I did not want to spend a great deal of time on this problem so I took to teh internet to see if I could discover where the condensate drain was to clean it out. But it seems my problem might a bit more complicated than I thought.
First I learned about the lack of a screen behind the air intake which lead me to check the air filter located behind the glove box. On top of the air filter I did indeed discover acorns, pine needles and (ew!) mouse droppings. I carefully removed the air filter and replaced it with a fresh one. I cut and make my own Yaris cabin filters and you'll find the step by step instructions for making a Yaris cabin filter here.
The next thing I learned is that there is a Technical Service Bulletin out for this year Yaris, apparently the metals used in the air conditioning condenser can generate a particular musty smell and cars under warranty qualify for a condenser replacement if the have this problem. If your car is still under warranty and this is the problem you definitely want to make Toyota fix it. Take your car in and cite TSB 0103-09 – More info here.
But the following video was the information I needed, the location and method for accessing and cleaning the Yaris' HVAC condensate drain hose. Three cheers for people that post simple but incredibly useful stuff like this!
However, when I took my hose off I found that the drain was clear, three was no blockage. This meant that there was only a small amount of organic material in my Yaris' HVAC system in which the mold and mildew were growing. I did not want to disassemble the entire dashboard to clean it out so I fill the largest syringe I could find with a mixture of 10% bleach and water and injected it into the HVAC housing through the condensate drain hose.
I injected 60 CCs and found that I could draw it all out again so I flushed the HVAC box several times with the bleach solution and rocked the car back and forth to splash the bleach around inside the air case. Once I was done with that I put the hose back and drizzled about 20 CCs of bleach solution down the left and right sides of the defroster ducts in an attempt to kill any remaining mildew.
These efforts seem to have worked as the musty smell in my Yaris is gone!