The air filter on a Hitachi projector keeps the fan from sucking dust in from the outside. You need to clean the filter at regular intervals, and the projector will remind you to clean it by displaying a prompt after a certain number of hours. After you have cleaned the filter, you need to reset the filter timer to get the prompt to go away.
Press the "MENU" button to access the main menu on the projector. You can use the button on the remote, or the one on the projector unit.
Highlight "OPTION" using the up and down arrow, and then press the right arrow to open the sub-menu.
Select "FILTER TIME" and hold down the "RESET" button until a screen showing the time on the filter appears.
Press the up arrow to select "RESET."