HIGH URGENCY

Lennox 1 Flash — Flame Sensed With Gas Valve Off

1 flash

Flame detected when gas valve should be closed.

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

What This Code Means

1 LED flash on a Lennox ML or EL series furnace means the flame sensor detects flame even when the gas valve is de-energized. Usually a leaking gas valve.

Likely Causes (Ranked)

Most common

Leaky gas valve

What: Valve doesn't fully close.

How to check: Tech valve test.

Fix: Replace gas valve.

Shop parts

Less common

Faulty control board

What: Board falsely reports flame signal.

How to check: Tech diagnostic.

Fix: Replace board.

Shop parts

Diagnostic Steps

Shut off gas immediately
Cut power
Call licensed HVAC tech

Frequently Asked Questions

Is Lennox 1 flash dangerous?

Yes — uncontrolled fuel/flame. Shut off gas immediately.

DIY fix?

No — gas valve work is professional.

Same on Armstrong/Ducane?

Yes — Lennox family shares the same control system.

Related Lennox Codes

Related Symptoms

Need More Help?

Try our interactive symptom diagnostic tool or browse Lennox parts.

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