Originally Posted by Bill B.
I'll add another datapoint to this discussion. Sometimes the AC performance can decline very slowly and might not be very noticeable. If you have access to a thermometer, check each side of the vents with the AC on high. If there is any more than a slight difference in values, you probably have low refrigerant/leaking evaporator. The leak will cause a bad odor.
that's exactly what happened to my car, that's exactly what I was trying to explain above... it can take a year before it get noticeable...