Lennox Furnace Error Code E 240

E 240 means the control board detected low flame current from the flame sensor during an active heating cycle. The flame sensor uses a micro-ampere signal to confirm that the burner is lit, and if that signal drops below the required threshold, the board logs this alert. Common causes include a dirty sensor rod, a wiring ground issue, or reversed polarity at the electrical supply. The alert clears automatically after the current heat call is completed.

E 240 itself is a performance and reliability alert rather than an immediate safety shutoff, but it should not be ignored. Low flame current means the furnace has reduced confidence that the burner is properly lit, which can lead to nuisance lockouts or inconsistent heating. Addressing the root cause promptly prevents the situation from escalating to a full no-heat condition.

If the fix is only cleaning the flame sensor, costs are minimal, typically just a service call fee in the range of $75 to $150. If the sensor needs replacement, parts are generally in the $15 to $50 range with labor adding $100 to $200 depending on your area. Correcting a grounding or wiring issue will vary more widely depending on the complexity of the electrical work required.

Cleaning the flame sensor is a task many experienced DIYers can perform: locate the sensor rod in the burner assembly, disconnect the wire connector, and gently clean the rod with fine steel wool or emery cloth. However, checking micro-ampere output using control diagnostics and measuring neutral-to-ground voltage requires appropriate test equipment and electrical knowledge. If cleaning does not resolve the issue, or if a grounding or wiring problem is suspected, a qualified technician should complete the diagnosis.

The E 240 alert is designed to clear after the current heat call has been completed, not at the moment of a manual reset. Simply cycling the power may temporarily clear the display, but if the underlying cause such as a dirty sensor or grounding issue is not corrected, the code will return during the next heating cycle.