Lennox Furnace Error Code 5

Error Code 5 on a Lennox furnace indicates a Watchguard lockout due to the burner failing to ignite after the maximum number of retries or recycles. The furnace control board monitors each ignition attempt and, if combustion is never confirmed by the flame sensor, it shuts the system down for safety. This code points to a problem in the ignition sequence involving the ignitor, flame sensor, gas valve, or gas supply. The furnace will remain locked out for approximately 60 minutes before attempting to restart.

Error Code 5 means the furnace has already shut itself down as a safety precaution, so it is not actively running. The Watchguard lockout is designed specifically to prevent repeated failed ignition attempts that could cause unsafe gas accumulation. Do not attempt to repeatedly force restarts without identifying the root cause. If you smell gas in your home, leave immediately and contact your gas utility before troubleshooting.

Repair costs depend on which component has failed. Flame sensor cleaning is a low-cost service call, typically in the range of $80 to $150 including labor. Replacing a hot surface ignitor generally costs between $100 and $250 for parts and labor. A faulty gas valve replacement can run from $200 to $600 depending on the model and labor rates. Control board replacement is typically the most expensive repair, ranging from $300 to $700 or more.

The furnace will automatically attempt to reset after the 60-minute Watchguard lockout period expires. You can also reset the furnace manually by turning the thermostat off and back on or by cycling the power at the breaker. However, if the underlying problem is not corrected, the furnace will fail to ignite again and return to lockout. Resetting without diagnosis and repair is only a temporary measure.

Start by waiting for the 60-minute lockout period to expire, then inspect the hot surface ignitor for cracks and test its resistance. Clean the flame sensor rod with fine steel wool to remove oxidation buildup. Confirm that the gas supply valve is open and that gas pressure is adequate. If those steps do not resolve the issue, the gas valve or control board may need further testing or replacement by a qualified technician.