Why do my car vents smell bad when I turn on the AC?

The air filter in your car doesn't filter the interior (cabin) air; it just filters air going into the engine for combustion purposes. Some cars have a totally separate CABIN air filter to keep dust, etc. from coming through the vents.

The air conditioning compressor generates some moisture due to condensation... this moisture can form a small amount of mildew at times, and that's the smell that kicks in as your fan is first coming on. A quickie fix... spray some air freshener into each vent opening, then run your HEATER full blast for a few minutes...it should dry up the mildewed areas inside the vents and freshen the air for you. Check to see if your vehicle has a "CABIN" air filter that filters the incoming air... a dealer or Auto Zone, Advance auto parts, etc. can tell you if you have one and where it is. If you haven't changed it, I'd suggest doing so.

Hope this helped.

If you have an AC filter, go ahead and change it again. When you run AC very cold, the moisture from the outside air will condense in the duct. This creates the environment for mold and mildew to grow, especially when you are not using the car. Before you park for the night, run the heat until, um, it's damn hot in the car. Usually in the last minute or two of driving before getting home, I will switch to high heat, windows down of course. Sometimes you can see vapor accumulate on the windshield that was once in the duct.

Keep the dog and food out of the car too.

Mold or mildew build-up in the ducting are the likely causes. It's possible (but not likely) that something died in your A/C system. It's also possible that an animal peed on your air filter or a squirrel has stuffed your filter box full of dog kibble (I've seen it happen).

Take the car to a mechanic; it is not normal for the car to smell, and no system from any major marque smells that way when it's new from the factory (no, not even Ford). The mechanic can inspect your system and tell you what's wrong better than anyone online can guess.

Many things could be the cause -mold, mildew -once I pulled a dead rat out of the plenum in the dash of my Camaro. If it doesn't smell like anything dead, just get a can of deodorizer for A/C systems at your local auto parts store, which you spray into the air inlet at the base of the windshield. Anything more than that will require disassembly and cleaning of the offending parts.

go with answerman. What year and model car do you have. Most likely if you have a cabin filter it has never been changed since most garages and mechanics just started to realize they are there and very easy to change for big bucks. I own and operate 3 quick lubes and we probab ly sell about 30 cabin filters a week. Sometimes we find mouse nests in them sometimes they are just dirty but most people can tell the difference when we cahnge them. If your car doenst have one then good luck but I really dont know.

Built up condensation=mildew, mold...

Let it blow for a while (a few trips) and see if it continues. If it does you can take it to a service station for an estimate/guesstimate.

Hope this helps.

You must be driving a Ford. One of my relatives used to own a Ford Taurus and it smelt bad when the A/C was on. I guess it has to do with the material used when building the car.

IF the Drain on the Evaporator is clogged or partially you will get the smell you are describing.Make sure the water is running steady[out of] of the Evaporator.

i believe its because the vent has many airways and as you start the car the smell from the engine goes tru the vent and as from there to you...

i belive there is a dead mouse inside it

cheack it out and clean it

but if it smells exhust ,so u must stop driving it and fix it ,it's too danger for u