Lennox E273 Error Code

What does Lennox E273 mean?

E273 is a soft lockout triggered when the furnace has exceeded the maximum number of low flame current events during run mode — the burners light and run but the flame sensor current drops below the threshold repeatedly across multiple calls. After exceeding the retry count, the board enters Watchguard mode. This is the soft-lockout escalation from repeated E240 conditions.

Symptoms

• E273 displayed on the thermostat or control board; furnace enters Watchguard mode.
• E240 faults appear in the fault history preceding the E273 lockout.
• Furnace produces heat intermittently across multiple cycles before locking out.
• Flame current below 1.0 µA confirmed in board diagnostic mode during prior cycles.

Common causes

• Heavily oxidized or silicone-contaminated flame sensor rod producing consistently low current across multiple heating cycles.
• Poor unit grounding elevating neutral-to-ground voltage and reducing apparent flame sensor signal.
• Burner flame quality degraded by low gas pressure or partially blocked orifices causing persistent low flame current.
• Flame sensor rod position shifted away from the flame envelope, reducing ion current.

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Diagnostic steps

1. Reset the Watchguard lockout

   Power cycle the furnace — turn the disconnect off for 30 seconds, then restore. This clears the E273 soft lockout and allows another ignition attempt.

2. Clean the flame sensor rod

   Remove the flame sensor. Lightly abrade the stainless steel sensing rod with fine steel wool or emery cloth to restore conductivity. Avoid silicone-containing abrasives. Reinstall and confirm the sensor is fully seated in its bracket.

3. Verify unit grounding

   Measure neutral-to-ground voltage at the furnace with a multimeter. Readings above 0.5V indicate a grounding issue that artificially reduces flame current. Verify the furnace is bonded to a proper earth ground.

4. Measure flame current in diagnostic mode

   Use the IFC board diagnostic mode to read live flame sensor microamperes with burners firing. Confirm current is above the minimum threshold after cleaning. If current remains below 1.0 µA with a clean sensor, gas pressure or burner condition is the cause.

5. Replace flame sensor if cleaning does not restore current

   If flame current remains below the minimum threshold after thorough cleaning and grounding is confirmed correct, replace the flame sensor with the Lennox OEM part.

When to call a professional

If flame current is confirmed adequate with a clean sensor but E273 returns, the IFC board's flame sensing circuit may have drifted or the gas combustion quality is poor. A licensed technician can measure gas manifold pressure, perform combustion analysis, and use factory diagnostic tools to verify the flame signal pathway through the IFC board.