Typically, there are two filters in a vehicle. One is for the engine and the other is for the cabin. A dirty filter can prevent airflow, limiting the engine's ability to expend hot air and inhibiting clean air to pass through. How often you need to change it varies, though the conventional timeframe is about 30,000 miles, which is why the best way to determine that it needs a replacement is to check it periodically. Fortunately, replacing a filter is a quick and easy process.