Lennox HVAC Error Code 5 Flashes

A 5-flash error on a Lennox HVAC system means the control board has detected a flame signal while the gas valve was not commanded open. This is treated as an unsafe condition and the system shuts down as a precaution. The fault points to a possible leaking gas valve, a defective flame sensor, or a wiring issue in the flame sensing circuit.

It can be. If the code is triggered by a gas valve that is leaking or stuck open, there is a risk of uncontrolled combustion inside the furnace. The system shuts itself down specifically to prevent unsafe operation. Until the cause is confirmed, treat the situation as a potential gas-related hazard and ventilate the area if you detect any gas odor.

A power reset may temporarily clear the fault and allow the furnace to attempt a new startup cycle, but if the underlying cause is still present, the error will return. Resetting without diagnosing the root cause is not recommended, especially if a gas valve leak is involved. The error should be properly diagnosed before the system is returned to regular operation.

Some causes, such as a shorted wire or a dirty flame sensor, may be within reach of an experienced DIYer comfortable working on furnaces. However, if the fault is caused by a leaking gas valve, this is a safety-critical repair that should be handled by a licensed HVAC technician. Do not bypass or ignore this error code before identifying the root cause.

Repair costs vary depending on the cause. Flame sensor replacement is typically a lower-cost repair, generally in the range of $75 to $200 including labor. Gas valve replacement is more involved and can range from $200 to $600 or more depending on the model and labor rates in your area. Control board replacement tends to fall in a similar or higher range.