Lennox Furnace 5 error code
Five flashes are the Watchguard response to repeated ignition failures: the control locks the furnace out for one hour before it will try again.
Also shown as: 5 flashes, five flashes, fault code 5, watchguard, ignition lockout
Last updated: September 30, 2026
What causes the 5 error?
- Persistent ignition failure (see 3-flash causes)
- Gas supply problem
- Failing igniter or flame sensor
How to fix Lennox 5 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Fix the underlying cause first
Work through the 3-flash checks: gas valve open, clean filter, clear vents, thermostat calling for heat. - Wait out the hour or power-cycle once
The lockout clears automatically after an hour. A single power cycle at the service switch also resets it — but only after the cause is addressed. - Call a technician
If the furnace locks out again on the next call, the ignition system needs professional diagnosis.Safety warning: Do not repeatedly reset a locking-out furnace. Continued failed ignition attempts wear out the igniter and can stress the heat exchanger.
When to call a technician
- The lockout recurs on the next heating call
- You smell gas — leave and call your gas utility
Frequently asked questions
Why does it wait a full hour?
Watchguard is a safety cool-down: it prevents endless ignition attempts while giving a transient problem (like a brief gas interruption) a chance to clear.
Can I shorten the wait?
One power cycle resets it, but resetting without fixing the cause just starts the failure cycle over.
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.