Lennox Furnace 11 error code
Eleven flashes mean the pressure switch was already closed before the inducer started — it is stuck closed, or its wiring is shorted.
Also shown as: 11 flashes, eleven flashes, fault code 11, pressure switch stuck closed
Last updated: September 30, 2026
What causes the 11 error?
- Pressure switch contacts stuck closed
- Shorted switch wiring
- Failed switch diaphragm
How to fix Lennox 11 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Power-cycle once
Turn the furnace off for 30 seconds and back on. A temporarily stuck switch may release. - Call a technician
The switch must be tested and replaced if it will not open. Wiring shorts need professional repair.Safety warning: Do not jumper or bypass the pressure switch to 'test' it — this defeats a critical venting safety check.
When to call a technician
- The code returns after a power cycle
Frequently asked questions
Is stuck-closed as bad as stuck-open?
Yes — either way the control cannot trust the venting proof, so it refuses to run the burners.
What makes a switch stick closed?
Age, moisture, or debris in the switch can keep the contacts from opening when pressure drops.
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.