Odor is a normal characteristic of automotive air conditioners. However, the odor's intensity can be reduced significantly by identifying and eliminating its root cause. An unpleasant odor emanating from the dash vents is commonly caused by mice or other small animals nesting in the air box -- which requires removing the nest from the inside of the duct or blower motor area -- or it can be caused by mold and bacteria growing on or around the air conditioner's evaporator core.
Step 1: Unclog the Evaporator Drain Tube
The most common contributor to mold and bacteria growth in the air box is a clogged evaporator drain tube. The drain tube should steadily drip water when the air conditioner is running, especially when it's humid outside. If this drain tube is clogged with leaves or other debris, the evaporator's condensation won't drain out causing water to stagnate. Find the drain tube by looking under the passenger side of the vehicle near the bottom of the firewall area. A metal coat hanger works well to gently push through the clog.
Water and debris can rush out of the drain tube when it is unclogged. Safety glasses are recommended for this under-car procedure.
The evaporator core can be punctured or damaged very easily by pushing the coat hanger too far. Do not allow the hanger to travel past the drain tube.
Step 2: Clean the Evaporator Core
If the odor continues after unclogging the drain tube, then you may need to use an anti-odor kit. Several products are available for this purpose. One product that can be purchased is a spray can with a long hose to reach through the vents and kill the mold and bacteria. This is the easiest method but not always the most effective. Other products are available, but they may require access to the evaporator core by either removing the blower motor, removing the blower resistor or by drilling a hole in the duct near the evaporator. If you buy the kit, follow the step-by-step manufacturer's instructions. These products typically use a chemical cleaner that is sprayed onto the evaporator and allowed to dry. You then apply another chemical to prevent further mold and bacterial growth.
Step 3: Replace the Cabin Filter
Many cars today use a cabin air filter to clean the incoming air. These filters typically need to be replaced every year or every 10,000 to 15,000 miles, according to vehicle manufacturers. A charcoal impregnated cabin filter can be purchased to help the elimination of odors. Access to replace the cabin filter may be from under the hood near the firewall, under the windshield or from behind the glove box. It is housed inside the air box.
Specific instructions explaining the replacement interval and procedure of the cabin air filter can be found in the vehicle's owner's manual.
To help prevent future mold and bacterial growth, when parking the vehicle set the HVAC system to the outside -- fresh-air -- mode. This can help dry out the air box.