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Lennox Red fast flash, green slow flash: Main power polarity reversed. Switch line and neutral

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

Status codeRed fast flash, green slow flashLennox
What it means

Main power polarity reversed. Switch line and neutral.

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How it shows: Red LED #1: fast flash; Green LED #2: slow flash

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§1Likely causes

  • Line and neutral reversed

§2Safe DIY steps

  1. Technicianswitch line and neutral
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: line-voltage wiring must be corrected by a qualified person.

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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 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.

Red slow flash, green fast flashLennox

Low flame signal: measures below 1.5 microamps. Replace flame sense rod.