Lennox Furnace Error Code 5 Flashes (RAM)

The 5 Flashes (RAM) error means the furnace control board detected a flame signal at a time when the gas valve was not commanded to open. This is treated as a safety fault and will prevent normal furnace operation. The two most likely causes are a gas valve that is not closing properly or a flame sensor that is producing a false signal.

The fault can indicate a gas valve that is not sealing properly, which is a condition that warrants prompt attention. If you suspect the valve is leaking gas between cycles, turn off the gas supply to the furnace and contact a qualified technician before attempting further diagnostics. A false flame signal from a faulty sensor is less immediately hazardous but should still be corrected before running the furnace.

Repair costs vary depending on which component has failed. Flame sensor replacement is generally one of the less expensive repairs, with parts and labor typically ranging from $75 to $200. Gas valve replacement is more involved and can range from $200 to $600 or more depending on the valve model and labor rates in your area. Control board replacement, if needed, typically falls in a similar or higher range than a gas valve.

Start by inspecting the gas valve to confirm it closes fully and operates correctly when de-energized. If the valve is functioning normally, remove and inspect the flame sensor rod for carbon buildup, cracks in the ceramic insulator, or shorted wiring. Cleaning or replacing a contaminated sensor is often sufficient to resolve the fault, but a stuck gas valve will require replacement.

A power reset may temporarily clear the lockout and allow the furnace to attempt another startup cycle, but if the underlying cause is still present, the fault will return. Resetting the unit without addressing the gas valve or flame sensor does not resolve the root problem. Repeated resets without repair can mask a condition that requires professional service.