Lennox Furnace Error Code LED1-OFF/LED2-SLOW_FLASH

This code indicates a pressure switch fault. The pressure switch monitors whether the combustion air blower is generating adequate airflow before ignition is allowed. The fault can appear because the switch failed to close during startup, opened during operation, opened five or more times in one heating cycle, or was already closed before the inducer turned on. Common triggers include blocked vents, a clogged condensate line, or a faulty pressure switch.

No. This fault code causes the furnace to shut down the ignition sequence intentionally, meaning it will not produce heat while the fault is active. Attempting to repeatedly reset the furnace without identifying the root cause does not make it safe to operate. The pressure switch system exists to prevent combustion without proper airflow, and bypassing or ignoring this fault creates a risk of incomplete combustion or carbon monoxide production.

Repair costs vary depending on the cause. Clearing a blocked vent or condensate line may cost relatively little if no parts are needed, typically in the range of a standard diagnostic service fee. Replacing a pressure switch generally falls in the range of $100 to $300 including parts and labor. Inducer motor replacement is more involved and can range from $300 to $700 or more depending on the specific model and local labor rates.

Some causes can be addressed by a homeowner, including clearing visible blockages at exterior vent terminations and checking the condensate drain line for obvious clogs. However, testing the pressure switch with a manometer, inspecting internal hoses, and evaluating the inducer motor require tools and skills typically held by a trained technician. If basic visual checks do not reveal the problem, professional service is recommended.

A reset may temporarily clear the fault and allow the furnace to attempt another startup cycle, but if the underlying cause is not corrected, the error will return. Because this fault also triggers after the pressure switch opens five times in a single call for heat, repeated resets without a fix will cause the fault to reappear quickly. Identifying and correcting the root cause is necessary before the furnace will operate reliably.