Lennox Furnace Error Code 5 Flashes

Five flashes on a Lennox furnace indicate that the rollout switch (control 103085) has opened. This safety switch trips when it detects flames escaping outside the combustion chamber, which can result from blocked flue passages, heat exchanger problems, or restricted combustion airflow. The furnace will shut down and remain off until the root cause is corrected and the switch is manually reset.

Start by inspecting the combustion passages, burner ports, and the full flue venting system for any blockage or obstruction. Once you have identified and corrected the cause, the rollout switch must be manually reset before the furnace will operate again. If you cannot identify a clear cause, or if the heat exchanger is suspected to be cracked, contact a qualified HVAC technician before operating the furnace.

Repair costs vary significantly depending on the cause. If the issue is simply a blocked flue or dirty burner passages, a cleaning and service call may cost between $80 and $200. If the heat exchanger is cracked and requires replacement, costs can range from $500 to $1,500 or more depending on the unit and labor rates in your area. A technician will need to diagnose the specific cause before an accurate estimate can be provided.

Yes, this is a serious safety condition. Flame rollout means combustion is occurring outside the designed containment area of the furnace, which can damage components, wiring, and nearby materials. Do not repeatedly reset the rollout switch without identifying and fixing the cause, as doing so defeats the protection the safety device provides.

Resetting the rollout switch will allow the furnace to attempt operation again, but the 5 Flashes error will return if the underlying problem has not been resolved. The switch is designed to require a manual reset precisely so that a technician must physically inspect the unit before it is returned to service. Clearing the code without fixing the root cause is unsafe.