We may earn a commission from links on this page. Learn more about affiliate links

Lennox Alternating slow flash: Watchguard 1 hour: burners failed to ignite or lost flame 5 times during a single heating demand

Ref. LENNOX-ALTERNATING-SLOW-FLASH · From the Lennox furnace manual · Updated 2026-10-23

Status codeAlternating slow flashLennox
What it means

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

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

Find a local HVAC pro →

§1Likely causes

  • Burners failing to ignite
  • Flame lost repeatedly

§2Safe DIY steps

  1. Confirm the gas supply to the furnace is on
  2. Techniciandiagnose ignition and flame sensing
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 furnace enters Watchguard again.

Find a local HVAC pro →

§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.

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.

Red slow flash, green fast flashLennox

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