For a gasoline engine to properly combust fuel, a precise ratio of fuel to air is needed. Too little air in the mixture can force the engine to work harder than it should have to, and unclean air can cause problems as well. This is why every internal combustion engine is equipped with an air filter – to keep dirt and other debris from getting into the system and mucking things up while ensuring that the proper amount of air reaches the combustion chamber.
Air filters work hard to keep the engine clean, and you should keep an eye on them to make sure they can do their job effectively. Clogged air filters can negatively impact a car's performance and possibly shorten its lifespan. If the filter is dirty enough to unbalance the air-to-fuel ratio, it may cause the engine to run roughly and the Check Engine light could light up on your dash panel.
You can consult the owner's manual of your vehicle to determine the recommended intervals of replacing the filters, but keep in mind that exterior factors, such as environment and how often and how hard you drive your car, can increase or decrease the amount of time an air filter will be effective. For instance, if you drive down a lot of dusty dirt roads your car's air filters may need to be changed more frequently than if you spend most of your driving time on highways.
How Stuff Works states that some experts recommend checking the air filter when you get your oil changed, which is recommended every 3,000 miles, but it is actually very specific to each individual driver. Other sources say that filters can last up to a year or even 30,000 miles, so you should take your location and driving habits into consideration when deciding how often to check the air filter. If you are unsure, you might want to err on the side of caution and check it more frequently.
To check the air filter, remove it from the engine and hold it up to the light. If it's obviously packed with dirt, you can replace the auto part, but Yahoo Autos states that some dirt in an air filter is not necessarily a bad thing. The particles that are present in a slightly-used air filter can actually help trap some of the smaller bits that are trying to get through, which would otherwise be able to slide right past the filter's fibers.