Lennox E240 Error Code

What does Lennox E240 mean?

E240 indicates the flame sensor is measuring low microampere current during the run period — the burners are lit but the sensor signal is too weak for confident flame confirmation. The IFC monitors flame current continuously; a reading below the minimum threshold during run mode logs E240. The alert clears at the end of the current heat call. If flame current stays low, the system will eventually lose flame and escalate to a lockout.

Symptoms

• Furnace runs but E240 alert is logged — heat is produced but flame sensor current is marginal.
• Flame current below 1.0 microamperes as read on control board diagnostic mode.
• Furnace may intermittently lose flame and re-ignite during a long heating cycle.
• Alert clears at the end of the heat call but returns on subsequent calls.

Common causes

• Contaminated flame sensor rod with oxidation, carbon, or silicone coating reducing conductivity.
• Poor unit grounding causing high neutral-to-ground voltage that reduces apparent flame current.
• Flame sensor positioned incorrectly relative to the burner flame envelope.
• Weak flame due to low gas pressure or partially blocked burner orifice.
• Aging flame sensor rod with degraded surface conductivity.

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Diagnostic steps

1. Clean the flame sensor rod

   Remove the sensor, lightly abrade the stainless steel rod with fine steel wool or 400-grit emery cloth to remove the oxide or carbon coating, then reinstall. Do not use sandpaper with silicone content — silicone contamination permanently degrades the sensor.

2. Measure flame current in diagnostic mode

   Use the IFC board diagnostic mode to read flame sensor microamperes while the burners are lit. Typical acceptable values are 1.5–6 µA; below 1.0 µA confirms the low current condition.

3. Check unit grounding

   Measure voltage from the furnace neutral wire to earth ground. Readings above 0.5V indicate a grounding problem that artificially reduces the flame sensor signal. Ensure the furnace is properly bonded to ground.

4. Verify gas pressure and burner condition

   Measure manifold gas pressure with the burners firing and compare against the SL280DFNV nameplate specification. A weak flame from low gas pressure or dirty orifices produces low flame sensor current.

5. Replace the flame sensor if cleaning does not restore current

   If flame current remains below threshold after cleaning and grounding is confirmed correct, install the Lennox OEM replacement flame sensor.

When to call a professional

Gas pressure measurement and burner manifold access require a licensed technician with a calibrated manometer and gas handling credentials. If cleaning the sensor and verifying ground do not restore adequate flame current, call a technician to check gas pressure, inspect heat exchanger condition, and confirm the burner assembly is producing a proper flame envelope.