Lennox Furnace Error Code 4+6

Error code 4+6 on a Lennox furnace is a Watchguard fault indicating that a flame was sensed out of sequence, meaning the control board detected a flame signal when no ignition was commanded, or a flame failed to establish correctly during a startup attempt. This triggers a safety lockout that prevents furnace operation for 60 minutes, after which the unit will automatically retry. Repeated occurrences point to a fault in the flame sensing circuit, gas valve, or control board.

Yes, this fault involves the Watchguard safety system, which is designed to prevent unsafe combustion conditions. A flame sensed out of sequence could indicate a gas valve that is not closing properly or a control board misreading sensor signals, both of which are conditions that should be evaluated by a qualified technician. Do not bypass the lockout or attempt to override the safety controls.

Start by verifying that the flame sensor rod is clean and free of oxidation or residue buildup, as a dirty sensor is one of the most common causes of flame sensing faults. Next, inspect the flame sensor wiring and connector for damage or loose connections. If the sensor and wiring appear to be in good condition, the gas valve and control board should be tested by a qualified technician with the appropriate instruments.

The 4+6 Watchguard fault includes an automatic 60-minute retry, so the furnace will attempt to restart on its own without a manual reset. You can manually reset the control board by cycling power to the furnace, but if the underlying cause is not corrected, the fault will return. Clearing the code without addressing the root cause is not a long-term solution and may result in repeated lockouts.

Repair costs vary depending on the failed component. A flame sensor replacement is typically a lower-cost repair, often in the range of $75 to $200 including labor. A gas valve or control board replacement will generally run higher, potentially $300 to $700 or more depending on the part and local labor rates. A technician diagnosis is the first step to determine which component is at fault.