The capacity of the Toyota air filter to remove debris from the air that it is supplying the engine is finite. There is a limited time before the obstructions it collects inhibits the flow of air through it. The time frame in which the air filter and cleaner box should be inspected is really dependant on the air quality the vehicle normally operates in. For the many Toyota work trucks that operate on old dusty roads, the inspection and replacement of the air filter is more frequent that the Toyota cars that are mainly driven on the highways. In an inspection of the air filter, the amount of light that can be observed penetrating the paper element should be noted. On a new filter element, there will be a consistent level of light that shines through. As the debris fills in the pores of the Toyota air filer, the level of light that can be seen through the filter diminishes. On a used filter element, there will also be an inconstancy to the light in different sections of the filter that can be observed. The onboard computer will sense the amount of air that is flowing through the air filter cleaner on a constant basis. As the filter element becomes clogged with debris, the computer will compensate for the diminished air flow. There is a level at which this compensation of the diminished air flow can no longer take place and the performance of your Toyota engine will be negatively affected until a new air cleaning element is put into place on your vehicle. Not only does a dirty air cleaner affect the performance of your vehicle, but also the fuel mileage you can obtain when it is operating.