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Lennox Red slow flash, green off: Flame sensed without gas valve energized

Ref. LENNOX-RED-SLOW-GREEN-OFF · From the Lennox furnace manual · Updated 2026-10-23

Status codeRed slow flash, green offLennox
What it means

Flame sensed without gas valve energized.

Call a profurnace
How it shows: Red LED #1: slow flash; Green LED #2: off

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§1Likely causes

  • Flame present with gas valve de-energized

§2Safe DIY steps

  1. Techniciandiagnose gas valve and flame sense circuit
Safety firstIf you smell gas, do not touch any electrical switch or use a phone in the building; leave immediately and call your gas supplier from outside. Turn off electrical power at the furnace switch or breaker before opening any panel, and never bypass or jumper a limit, rollout or pressure switch. Gas valve, burner, venting, wiring and control-board work should be done by a licensed HVAC technician.

§3When to call a pro

Call a licensed HVAC technician: immediately; this is a gas safety fault.

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§4Other Lennox codes

Alternating fast flashLennox

Improper main ground, OR line voltage below 90 volts. Control will not respond to a heat call until voltage rises above 95 volts.

Alternating slow flashLennox

Watchguard 1 hour: burners failed to ignite or lost flame 5 times during a single heating demand.

Board failure patternLennox

Circuit board failure or control wired incorrectly.

Red fast flash, green slow flashLennox

Main power polarity reversed. Switch line and neutral.

Red off, green slow flashLennox

Pressure prove switch open, OR blocked inlet/exhaust vent, OR pressure switch closed before the combustion air inducer started.

Red on, green slow flashLennox

Rollout switch open, OR 12-pin connector improperly attached.