Lennox Furnace Error Codes — Complete Guide

Modern Lennox ML and EL series furnaces (and sister brands Armstrong Air, Aire-Flo, Ducane) use a single LED flash count code. Also covers Armstrong Air, Aire-Flo, Ducane.

All Lennox Error Codes

1 flash

Lennox 1 Flash — Flame Sensed With Gas Valve Off

Flame detected when gas valve should be closed.

2 flashes

Lennox 2 Flashes — Closed Pressure Switch With Inducer Off

Pressure switch stuck closed when inducer is off.

3 flashes

Lennox 3 Flashes — Open Pressure Switch With Inducer On

Pressure switch open during inducer operation — vent blockage or bad switch.

4 flashes

Lennox 4 Flashes — Primary Limit Switch Open

Primary limit switch open — overheating. Usually restricted airflow.

5 flashes

Lennox 5 Flashes — Roll Out Switch Open (SAFETY)

Roll out switch tripped — possible cracked heat exchanger and CO risk.

6 flashes

Lennox 6 Flashes — Pressure Switch System Lockout

Persistent pressure switch errors caused lockout.

7 flashes

Lennox 7 Flashes — Burners Failed to Ignite Lockout

Burners failed to ignite — locked out after retries.

8 flashes

Lennox 8 Flashes — Burners Lost Flame Too Many Times

Flame established but kept dropping out — usually flame sensor.

9 flashes

Lennox 9 Flashes — Line Voltage Polarity Incorrect

Line voltage polarity reversed — wiring or grounding issue.

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Lennox Furnace Error Codes — Complete Guide

All Lennox Error Codes

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