An air vent that may be where a car mildew smell is originating from.

It may seem that your car desperately needs an air freshener, but it is much better to root out the odor-causing source than to mask the off-putting funk. Frequent causes for musty smells include mold and/or mildew in the vent system, carpet, or headliner. Even a cabin air filter clogged with rotting leaves or leaking trunk can become odiferous. The common solution is to clean the area and treat with an anti-mildew solution.

“Beyond being unpleasant, bad smells can indicate a real problem, such as a clogged AC drain or leaking heater core, that needs to be repaired,” said John Ibbotson, CR’s chief mechanic.

If your nose leads to the vents, there are professional-grade spray products for cars that can can kill the fungi through the dash. Serious cases may require service at a dealership.

More on Car Maintenance

Pervasive odors can be traced to the carpet. A stinky floor can be caused by moisture collecting in the passenger footwell from a malfunctioning climate system, an open window, or snow brought in on feet. Steam clean the floor and treat with a carpet cleaner. Dry out the interior by using the air conditioner on recirculate when driving or by parking in the sun, with doors open, on a hot, low-humidity day.

As winter approaches, consider replacing your carpeted floor mats with rubber, all-weather mats for the season. And be sure to remove the regular mats, rather than stack the water-proof floor liners on top of them.

Trunk odors can be caused by cargo leaking or rain water seeping in around the seals. Again, dry the carpet out and treat with a rug shampoo. Replace the seals, if necessary.

Many problems are associated with older cars suffering from age and wear, but even newer cars can experience problems with sunroof drains being clogged with debris.

Plugged drains can cause water to leak onto the seats or travel along the headliner and lead to some puzzling problems and disturbing smells. Clearing the sunroof drain can be simple: Use a vacuum cleaner to draw out the debris and/or light air pressure to blow debris free. Resist the temptation to use a clothes hanger to clear the lines, as that risks tearing the tubing. And high-pressure air can dislodge the drain tubes.

Serious odors often have a significant cause that warrants a repair, but spills, trash, and drive-through leftovers can also pollute the cabin air. Routine maintenance and thorough car cleaning can avoid many problems, including unpleasant smells.

Editors Note: This article, which appeared in the October 2018 issue of Consumer Reports magazine, has been updated with additional information.

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Cars these days can run for 200,000 miles and more. On the 'Consumer 101' TV show, host Jack Rico learns from Consumer Reports' Jen Stockburger what you can do to get the maximum life out of your vehicle.