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 between the two sides, you probably have low refrigerant/leaking evaporator. The leak will cause a bad odor.
Last edited by Bill B.; 01-24-2013 at 08:15 PM.