Lennox Furnace Error Code 4+3

The 4+3 code on a Lennox furnace with control board 100869 indicates a Watchguard lockout. The furnace has exceeded the maximum number of ignition retry attempts, or the last retry failed due to a flame failure. The control board shuts the system down to prevent unburned gas from accumulating. The root cause is typically a flame sensing problem, a gas supply issue, or blocked burner crossovers.

The Watchguard lockout is itself a safety response, meaning the furnace has already stopped itself to avoid a potentially dangerous condition involving repeated failed ignition attempts. The furnace will not attempt to fire again until the fault is cleared. You should not attempt to repeatedly reset the furnace without identifying and correcting the underlying cause, as doing so bypasses the protection the Watchguard routine provides.

Repair costs depend on what component is at fault. Cleaning a flame sensor is a low-cost service call repair, typically in the range of $75 to $150 for labor. Replacing a flame sensor or sensor wire may add $20 to $80 in parts. A gas valve replacement is more involved and can range from $200 to $600 or more depending on the specific valve and labor time. Burner cleaning and crossover service generally falls in the lower to mid range of a standard diagnostic visit.

Some steps are accessible to experienced DIYers, such as cleaning the flame sensor rod, checking the gas shutoff valve position, and inspecting sensor wiring for loose connections. However, diagnosing gas valve problems or measuring gas pressure requires proper equipment and training. If basic checks do not resolve the issue, a licensed HVAC technician should perform further diagnosis to ensure safe operation.

Resetting the furnace by cycling power may clear the lockout temporarily and allow the furnace to attempt ignition again. However, if the underlying problem is not corrected, the Watchguard counter will reach its limit again and the 4+3 code will return. A reset without repairs is not a fix and repeated resets without diagnosis can mask a persistent problem with the flame sensor, gas supply, or burners.