Lennox Furnace 7 error code
Seven flashes mean a primary or secondary limit switch is open — the furnace overheated — or the control entered a Watchguard state after the limit stayed open for three minutes.
Also shown as: 7 flashes, seven flashes, fault code 7, limit open, high limit
Last updated: September 30, 2026
What causes the 7 error?
- Dirty air filter
- Blocked registers or closed-off ductwork
- Blower not moving enough air
- Faulty limit switch
How to fix Lennox 7 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Replace the air filter
Install a clean, correctly sized filter — this is the most common fix. - Open up airflow
Open all supply registers and make sure return grilles are not blocked by furniture or drapes. - Call a technician if it returns
Recurring limit trips need blower, ductwork, and limit-switch testing by a professional.
When to call a technician
- The code comes back with a clean filter
- The blower does not run during the fault
- You notice hot or burning smells around the furnace
Frequently asked questions
What is a limit switch?
A temperature safety switch on the heat exchanger that opens when the furnace gets too hot, shutting off the burners while the blower cools things down.
Can I keep running the furnace if it resets itself?
Occasional trips after a filter change are one thing; repeated trips mean the heat exchanger is being stressed and need professional attention.
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.