Lennox Furnace 3 error code
Three flashes mean the burners failed to light, or the flame was lost during the heating call. The furnace will retry and then lock out if the problem continues.
Also shown as: 3 flashes, three flashes, fault code 3, failed to light, lost flame
Last updated: September 30, 2026
What causes the 3 error?
- Gas supply shut off or interrupted
- Worn or broken hot-surface igniter
- Dirty flame sensor
- Blocked intake or exhaust venting
How to fix Lennox 3 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Confirm the gas valve is open
Check the manual shutoff in the gas line near the furnace — the handle should be parallel to the pipe. - Check thermostat, filter, and vents
Make sure the thermostat calls for heat, the air filter is clean, and the outdoor intake/exhaust pipes are clear of debris or snow. - Call a technician if it persists
Igniter, flame sensor, and gas pressure testing require a professional with a meter.Safety warning: If you smell gas, do not touch the furnace — leave the area and call your gas utility.
When to call a technician
- The code returns after the basic checks
- You hear clicking but the burners never light
- You smell gas at any time
Frequently asked questions
What is the clicking sound before the code?
That is the igniter or gas valve attempting the lighting sequence. Repeated clicking with no flame points to ignition trouble.
Will the furnace try again by itself?
It retries a few times, then enters a lockout (see the 5-flash Watchguard entry). Clear the underlying cause rather than just resetting.
Sources
Every meaning and fix above was checked against official manufacturer documentation:
FixCode provides general troubleshooting information only — not professional repair advice. If you are unsure about any step, stop and call a qualified technician. Repairs done incorrectly can be dangerous and may void your warranty.