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Lennox E240 - Low Flame Sensor Signal

E240

Low flame sensor signal - usually a dirty flame sensor.

Brand: Lennox (Also covers Armstrong Air, Aire-Flo, Ducane) | Equipment: Gas Furnace

What This Code Means

Error code E240 on Lennox iComfort means the flame sensor signal is weaker than expected. Usually precedes a full ignition lockout (E270) if not addressed.

Likely Causes (Ranked)

Most common (~80%)

Dirty flame sensor

What: Carbon or oxide buildup weakens signal.

How to check: Pull sensor; check for coating.

Fix: Clean with fine emery cloth.

Shop parts

Common

Low gas pressure

What: Insufficient gas produces weak flame.

How to check: Tech measures manifold pressure.

Fix: Adjust regulator or call gas utility.

Less common

Cracked igniter

What: Weak ignition produces unstable initial flame.

How to check: Watch glow brightness.

Fix: Replace igniter.

Shop parts

Diagnostic Steps

Pull flame sensor
Clean with emery cloth
Reinstall and cycle power
Watch flame quality
Call tech if persists

Frequently Asked Questions

How to clean Lennox flame sensor?

Cut power, pull sensor (one screw), wipe metal rod with fine emery cloth or steel wool, reinstall. 5-minute job.

Is E240 urgent?

Not critical, but address soon - usually progresses to E270 lockout.

Same on Armstrong/Ducane?

Yes.

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Need More Help?

Try our interactive symptom diagnostic tool or browse Lennox parts.

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