Lennox Furnace 4 error code
Four flashes mean the flame signal is too weak. The control sees a flame, but the sensing current is below the reliable threshold, so it may shut the burners down.
Also shown as: 4 flashes, four flashes, fault code 4, low flame signal, weak flame sense
Last updated: September 30, 2026
What causes the 4 error?
- Dirty or oxidized flame sensor rod
- Sensor rod not positioned fully in the flame
- Weak or partially blocked burner flame
- Poor ground connection
How to fix Lennox 4 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Check the basics
Confirm the gas valve is fully open and the air filter is clean so the flame burns normally. - Have the flame sensor serviced
A technician can clean the sensor rod, verify its position, and measure the flame signal strength.Safety warning: The sensor is inside the burner compartment. Do not reach in or attempt to clean it yourself — leave burner-area work to a qualified technician.
When to call a technician
- The furnace lights, then shuts off after a few seconds, repeatedly
- The 4-flash code appears on every heating call
Frequently asked questions
Can I clean the flame sensor myself?
It is a common technician task, but it involves opening the burner compartment and handling a delicate part near gas components — safest left to a professional.
Is a weak flame signal dangerous?
The shutdown is protective. But short-cycling stresses the heat exchanger over time, so get it serviced promptly.
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.