That small furnace flashing light behind the little window on your unit is trying to tell you something. Most modern furnaces use a blinking LED pattern as an error code that points to the problem, such as an ignition failure, a pressure switch issue, or a lockout. Sherwood Mechanical's guide shows you how to count the flashes, match them to the chart inside your furnace door, and write down the code before calling for furnace repair. It's a simple step that helps the technician arrive prepared.