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

Lennox Red slow flash, green fast flash: Low flame signal: measures below 1.5 microamps. Replace flame sense rod

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

Status codeRed slow flash, green fast flashLennox
What it means

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

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

Find a local HVAC pro →

§1Likely causes

  • Weak flame sense signal

§2Safe DIY steps

  1. Technicianreplace the flame sense rod
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: to replace the flame sensor.

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.

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.