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Lennox Red off, green slow flash: Pressure prove switch open, OR blocked inlet/exhaust vent, OR pressure switch closed before the combustion air inducer started

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

Status codeRed off, green slow flashLennox
What it means

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

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

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§1Likely causes

  • Blocked inlet or exhaust vent
  • Pressure switch open
  • Pressure switch closed prior to inducer activation

§2Safe DIY steps

  1. Check outdoor inlet/exhaust terminations for blockage
  2. Techniciancheck pressure switch and venting
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 vents are clear and the code persists.

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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 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.