The filter is important for the proper operation of a furnace. If the filter gets clogged, it will not allow the furnace to circulate properly, which can result in these symptoms. The blower door switch will automatically turn the unit off when the door to the filter is opened. You can simply replace the filter to save time and money! Make sure nothing is closer than 18 inches from all output vents and intake grilles. To ensure adequate airflow, NOTHING should be blocking them.