Lennox Furnace Error Code 4

Error code 4 on a Lennox furnace indicates a low flame signal from the flame sensor. The sensor rod is not sending the minimum required 1.5 microamp current back to the control board during burner operation. This causes the furnace to flag the error and may result in the burner shutting down. The issue is most often caused by a dirty sensor rod that needs cleaning.

The error itself is part of a safety system. The furnace is shutting down or alerting because it cannot confirm the burner flame is stable, which prevents unburned gas from accumulating. Running the furnace repeatedly through restart cycles without resolving the underlying issue is not recommended. Address the cause promptly to restore safe, reliable operation.

Resetting the furnace may temporarily clear the error and allow the furnace to attempt operation again, but it will not fix the underlying problem. If the flame sensor signal remains below 1.5 microamps, the error code 4 will return after the next ignition attempt. A reset is useful for confirming the error is consistent, but the sensor and related components must be inspected and repaired to permanently clear the code.

In many cases, yes. The first step is to locate the flame sensor rod in the burner assembly, remove it, and clean the metal rod with steel wool to remove any carbon or oxidation buildup. After reinstalling it, measure the flame signal with a multimeter capable of reading microamps to confirm it reads 1.5 microamps or greater. If cleaning does not resolve the error, further diagnosis involving gas pressure, ground connections, or component replacement may require a qualified technician.

If cleaning the flame sensor rod resolves the issue, your cost is minimal, typically just a service call fee if you hire a technician, which generally ranges from $75 to $150. Replacing a failed flame sensor rod is an inexpensive repair, with parts usually ranging from $10 to $30 and labor adding $50 to $150 depending on your area. If the ignition control board needs replacement, total costs can range from $300 to $600 or more including parts and labor.