Lennox Furnace Error Code LED1-SLOW_FLASH/LED2-ON

This code indicates that a primary or secondary limit switch has opened, which shuts down the burner to prevent overheating. The limit switch is a safety device that trips when internal temperatures exceed safe levels. Depending on the control board installed, the furnace will attempt ignition for either 5 trials or up to 3 minutes before entering a one-hour Watchguard lockout. The most common cause is restricted airflow through the system.

The limit switch resets on its own once the furnace cools to within its rated temperature range, so a manual power reset is not always necessary. However, if the board has entered the one-hour Watchguard lockout, the furnace will resume attempting ignition automatically after that period expires. Resetting without addressing the underlying airflow restriction will result in the error returning.

If the fix is simply replacing a clogged air filter, the cost is typically in the range of $10 to $30 for the part. A service call to diagnose and repair a faulty blower motor capacitor generally runs $75 to $200, while blower motor replacement can range from $300 to $600 or more depending on the motor and labor involved. Limit switch replacement is usually in the $100 to $250 range including parts and labor.

Start by checking and replacing the air filter, then inspect all supply and return registers to confirm they are open and unblocked. After the furnace cools down, the limit switch will reset automatically and the unit will attempt to restart. If the error returns repeatedly, have a technician inspect the blower motor, blower wheel, and limit switch itself for faults.

The furnace is designed to shut itself down when this code appears, so it will not continue running in an unsafe condition. You should not attempt to bypass or defeat the limit switch, as it protects against heat exchanger overheating and potential damage. Address the airflow issue before restoring normal operation.