Lennox Furnace Error Code 1+3

Error code 1+3 means the furnace control board detected a flame signal at a time when the gas valve had not been energized, which is described as flame sensed out of sequence. This is a safety lockout condition. The furnace shuts down to prevent unsafe operation until the root cause is identified and corrected.

A manual reset may temporarily clear the fault and allow the furnace to attempt another ignition cycle, but if the underlying cause is not corrected, the code will return. Repeated resets without diagnosis can mask a worsening problem, particularly if a leaking gas valve is involved. Diagnosis and repair should be completed before relying on a reset as a solution.

Repair costs vary depending on the failed component. Flame sensor replacement is typically a lower-cost repair, often in the range of $75 to $200 including labor. Gas valve replacement is more involved and can range from $250 to $600 or more depending on the model and labor rates. Control board replacement tends to fall in the $300 to $700 range. A diagnostic service call is usually required to identify the specific cause before parts costs can be confirmed.

You should not continue operating the furnace while this code is active. A 1+3 fault can indicate a gas valve that is not fully closing, which presents a potential combustion safety risk. Have the system inspected by a qualified technician before restoring normal operation.

Fixing this error requires identifying whether the fault is caused by a contaminated or failed flame sensor, a leaking gas valve, a wiring fault, or a defective control board. Start by inspecting the flame sensor for contamination and checking all wiring connections on the flame sense circuit for shorts or damage. If those components check out, the gas valve and control board should be tested by a technician.