Lennox Furnace Error Code 3

Error code 3 means the furnace experienced an ignition failure. The burner either did not light during a call for heat or the flame was lost after ignition was confirmed. The control board locks out the system and displays this code to prevent unburned gas from accumulating. Diagnosis should start with checking ignitor resistance, flame sensor condition, and gas valve operation.

The furnace shutting down on this code is itself a safety response, designed to prevent unburned gas buildup in the heat exchanger and flue. While the lockout condition is safe, you should not repeatedly reset the furnace and ignore the fault, as persistent ignition failures can stress components and may indicate a gas flow issue that warrants professional inspection.

Repair costs vary depending on the root cause. Flame sensor cleaning is typically included in a standard diagnostic service call, which can range from roughly $75 to $150. Replacing a hot surface ignitor generally costs between $100 and $250 including parts and labor. Gas valve replacement is a more involved repair and can range from $300 to $600 or more depending on the specific model and local labor rates.

Some causes are DIY-friendly, such as cleaning a dirty flame sensor with fine steel wool or verifying that the gas shutoff valve is fully open. However, testing ignitor resistance requires a multimeter and basic electrical knowledge, and gas valve diagnosis should be handled by a qualified technician. If you are not comfortable working around gas lines and high-voltage components, professional service is the safer choice.

A manual reset may temporarily clear the lockout and allow the furnace to attempt ignition again. However, if the underlying cause is not corrected, such as a failed ignitor or coated flame sensor, the furnace will lock out again and the code will return. A reset is useful for a one-time nuisance fault but should not substitute for a proper diagnostic.