Lennox Furnace Error Code LED#1 Slow Flash / LED#2 ON

This code means that a primary limit switch, secondary limit switch, or rollout switch has opened, indicating the furnace detected an overtemperature condition somewhere in the heat exchanger or burner area. The furnace shuts down the heating cycle to protect itself and your home. If the condition is not corrected within the specified number of trials or time period, the control board enters a one-hour Watchguard lockout mode that prevents further ignition attempts.

No, you should not attempt to force the furnace to run while this error is present. The limit and rollout switches are safety devices that protect against fire, heat exchanger damage, and carbon monoxide exposure. A tripped rollout switch in particular is a serious indicator of a potential combustion problem that requires professional evaluation before the furnace is operated again.

Repair costs vary widely depending on the root cause. Replacing an air filter is inexpensive, typically under $30. A failed blower capacitor usually runs between $75 and $200 including labor. A blower motor replacement can range from $300 to $600 or more. If a cracked heat exchanger is found, replacement costs can range from $500 to well over $1,500, and in some cases replacing the furnace entirely may be more cost-effective.

Start by checking and replacing the air filter if it is dirty, since restricted airflow is the most common cause of limit trips. Next, confirm that all supply and return registers throughout the home are open and unobstructed. If the filter and registers are fine, inspect the blower motor for proper operation and check rollout switches near the burner compartment for any signs of heat or flame rollout. A tripped rollout switch should not be reset without a thorough inspection of the heat exchanger and flue system.

Cycling power to the furnace may allow it to attempt another ignition sequence once the one-hour Watchguard period has elapsed or the limit switch has cooled and closed, but the error will return if the underlying cause is not corrected. Simply resetting the board without addressing the overtemperature condition will result in repeated limit trips. Identify and fix the root cause before relying on a reset.