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Lennox Board failure pattern: Circuit board failure or control wired incorrectly

Ref. LENNOX-BOARD-FAILURE · From the Lennox furnace manual · Updated 2026-10-23

Status codeBoard failure patternLennox
What it means

Circuit board failure or control wired incorrectly.

Call a profurnace
How it shows: Red/green: on/on, on/off, or off/on (steady)

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§1Likely causes

  • Integrated control failure
  • Control wired incorrectly

§2Safe DIY steps

  1. Reset the control by turning main power off and on
  2. Techniciancheck wiring and replace control if needed
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: if the pattern returns after a power reset.

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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.

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.

Red slow flash, green fast flashLennox

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