The heater, ventilation and air conditioning system in all modern vehicles consists of a series of ducts and passages that air flows through. Air flows through a grid (heater core) for the heater and another one for the air conditioner (evaporator core). The air conditioning grid also separates moisture from the air that passes through it. There is a drain to let this moisture out. Air is directed through the proper path by a series of doors. Some vehicles have a filter that is designed to filter out some of the dust and Pollutants. This works much the same as the air filter in our home. All of this is designed to make us comfortable while driving. Mold and mildew love a dark, wet, warm place to grow. All the passages inside this system are prime living quarters for it. The air conditioning grid gets wet when it is working and warm when the vehicle is stopped. This makes the air conditioning grid especially susceptible to the growth of mold. A musty smell inside the vehicle just as soon as it is started can mean mold has formed. If a musty smell is present before a vehicle has started it usually means there is another source for mold such as a wet carpet due to some type of water leak or a plugged air conditioning drain. Since the air we breath is very important to our health and well being we recommend the cabin air filter be replaced about every 15,000 to 30,000 miles if the vehicle is equipped. We also recommend a service to clean the mold and mildew be performed about every 2 to 3 years. This service will fill the ductwork around the air conditioning grid with a mold killing foam and then flush it out. This also cleans out the drain. If water has soaked the carpet or carpet padding this type service will make little to no difference in the smell. In this case the source of the water must be corrected and the wet carpet or padding be cleaned and dried or replaced.