Lennox Furnace Error Code 4+7

The 4+7 error code on a Lennox furnace indicates a Watchguard ignitor circuit fault. The control board has detected that the ignitor circuit is not operating correctly, which is most commonly caused by a failed ignitor or a wiring problem. The furnace will lock out and automatically retry ignition after 60 minutes. If the ignitor resistance does not measure between 25 and 47 ohms, the ignitor needs to be replaced.

The Watchguard lockout is designed to prevent the furnace from attempting repeated failed ignition cycles, which protects against unburned gas accumulation. While the furnace will not operate normally during the lockout, you should not attempt to bypass the safety system. Have the ignitor circuit inspected and repaired before returning the furnace to regular service.

The Watchguard function includes an automatic retry after 60 minutes, so the furnace will attempt to restart on its own. Manually resetting power to the furnace may also clear the fault temporarily, but if the underlying ignitor or wiring problem is not corrected, the 4+7 code will return. A lasting fix requires diagnosing and repairing the root cause in the ignitor circuit.

A knowledgeable DIYer can inspect the ignitor wiring and test the ignitor resistance with a multimeter to check whether it falls in the 25 to 47 ohm range. If the ignitor is out of spec, replacing it is a manageable task with the correct replacement part. However, if wiring or the control board is involved, or if you are not comfortable working around gas appliances and electrical components, a licensed HVAC technician should perform the repair.

If the repair requires only a new hot surface ignitor, parts and labor typically range from around 100 to 300 dollars depending on the specific ignitor and local labor rates. If the control board needs replacement, total costs can range from 300 to 700 dollars or more. Getting a diagnostic estimate from a licensed technician before committing to a repair is always a good idea.