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Having the A/C default to recirculate when the temp is above 72, is the main reason. Not sure when they started doing that, but it's been that way since at least 2013. It never switches to outside air on its own. I noticed the musty smell within the first few months of ownership. This forum has helped educate me on the "fix".
1: Set the system to outside air immediately, if it's hotter inside the car than the air outside. Otherwise, wait until the interior cools down and do it.
2: Or, turn off the A/C several minutes before parking and shutting down the vehicle. (Number 1 is much more practical)
I usually find #2 is the case: mold/mildew in the evaporator. Sometimes they're easy to clean out by running a cleaning solution up through the drain tube. Other times I have to pull apart the dash to get to the actual evaporator and clean out the junk. On older cars, you almost have to do this. Changing cabin filter alone won't do much. On newer cars, I always recommend people to clean their evaporator (via drain tube) at least once a year as a preventative maintenance. It's much easier to keep mold under control from the beginning.
4 Reasons Why the Air Conditioner Smells in Your Lexus
