Lennox furnaces are large, complex appliances that combine electric relays, motors, fans, gas burners and electronics to provide heat for a home. With so many parts to go wrong, it's not unusual for a Lennox furnace to need repair after several years. Fortunately, the company makes it easy to diagnose problems with its color-coded LED lights on the front panel.
Look for the code that explains what the sequence of blinking lights means. Explanations of codes are usually found in the manual. If not, look on the back of the furnace or inside the access panel.
Match up the flashing green light pattern with the Lennox code for that particular model of furnace. For example, six flashing lights in a row may mean the furnace has locked the starting mechanism because the gas burner attempted too many starts without success. A technician won't know the specific problem until he opens and examines the appliance, but he'll know where to start looking.
Don't use another furnace code. Other manufacturers use the same flashing light system but with different codes. If you use another system of codes, you may misdiagnose the problem.
Call the service technician if the flashing lights indicate a problem that's urgent, such as fire where it's not supposed to be or a malfunctioning gas valve. Don't attempt to fix these yourself.