Freshen the Air

• If the smell is not overpowering, you may be able to cover it up. You can use regular car air fresheners for this: just cut them into a shape and size that fits inside the vent and set them in there so they don't fall out. Alternatively, you can use dryer sheets. If you're in a hurry, try spraying a little perfume into the vents.
  
  

Clean the Vents

• Use soap and water, or mix 1/4 cup of white vinegar into 2 cups of water to get better results. If you can't stand the smell of vinegar while you're cleaning, then you can use antibacterial agents instead. Use an old toothbrush to scrub, or add the mixture to a spray bottle and wipe it on the vents with a rag.
  
  

Clean the Air Conditioner

• Bad odors can be a consequence of a dirty air conditioner. To fix this, take the cover off your air conditioning unit and use a damp cloth to wipe it down. Wipe inside all cracks, crevices, and drains, then remove the collection pan. This pan collects condensation from the air conditioner; over time, the pan becomes clogged with dirt and fungus. Clean it well before replacing it.
  
  

Change the Filter

• Changing your air filter takes care of vent odor in some cases. You can buy a new filter from any auto parts store. To change it, look for a big metal cylinder under the hood of your car. This is attached to a box where the filter goes, and it should easily come off so you can replace the old filter.
  
  

Other Tips

• Other things besides just dirt and moisture can cause bad odors. If the suggestions above don't seem to be working, consider these: if you smoke in your car, the tobacco can cling to the fabric of your upholstery, and eventually it makes its way into the duct system. Pet odors may also be causing a bad smell. And if your engine is leaking somewhere, you may be smelling the fumes from burning engine fluids. Get it checked out if you suspect this is the case.
  
  

Warnings

• Dryer sheets, and occasionally air fresheners, have begun receiving some negative attention in the media for the unhealthy chemicals they contain. If this is a concern to you, then skip the first suggestion and just begin by cleaning your vents.