Air Filter Replacement

Engine Air Filter is usually at the input of the air-intake system.  Air intake system in turn provides the necessary air (some times below atmospheric pressure) to the Fuel System's intake manifold. To put it simply , the engine needs air for burning and this is the filter it uses to make sure the air is free from dry leaves , dead insects and other debris. A good vs bad filter is shown below :

The filter is usually changed as per the miles in the user manual - usually 20K to 30K miles. For harsh weather or environment change it more often like every 3 oil changes.  See example of many kinds 0f car air filters here :

This filter is usually housed in a big rectangular block that is often black in color.  The box cover is usually held in place by four clips or such.  Press or pull the clip latch on all the clips and shake open the top cover of the air filter box. You will now see the air filter.  See the picture below for example of the filter housing :

Replace with similar type air filter. It is also OK to remove it clean it and install it back in its mid-life periods.

Wally and parts store does stock air filter for most popular american and japanses cars.  For a few European cars you might want to get this online or from the Parts section of the Dealership.