Lennox Furnace 9 error code
Nine flashes mean the pressure switch did not close when the inducer started, or it opened in the middle of a heating call — the control cannot prove safe venting, so the burners stay off.
Also shown as: 9 flashes, nine flashes, fault code 9, pressure switch fault
Last updated: September 30, 2026
What causes the 9 error?
- Blocked intake or exhaust venting
- Kinked, cracked, or water-filled pressure hoses
- Weak inducer motor
- Blocked condensate drain (on condensing models)
How to fix Lennox 9 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Clear the outdoor vent pipes
Remove leaves, snow, ice, nests, or debris from the intake and exhaust terminations outside. - Check the pressure hoses
With power off, look for kinks, cracks, or trapped water in the small rubber hoses to the pressure switch. - Call a technician
Draft pressure, the inducer, and the switch itself are tested with professional instruments.Safety warning: Never bypass a pressure switch. It proves combustion gases are venting outside — bypassing it creates a carbon monoxide hazard.
When to call a technician
- Vents and hoses look fine but the code persists
- The inducer runs but the code appears anyway
Frequently asked questions
What does the pressure switch prove?
That the inducer fan is moving combustion air and exhaust through the furnace properly. No proof, no flame — by design.
Can cold weather cause this?
Yes — ice or frost can block the outdoor pipes or freeze water in the hoses. Clear them and see if the code clears.
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.