Lennox Furnace 10 error code
Ten flashes are the Watchguard lockout for the pressure switch: it opened five times during a single call for heat, so the control locks the furnace out.
Also shown as: 10 flashes, ten flashes, fault code 10, watchguard pressure switch
Last updated: September 30, 2026
What causes the 10 error?
- Intermittent vent blockage (wind, debris)
- Failing inducer motor
- Pressure switch sticking intermittently
- Condensate draining intermittently into the switch tubing
How to fix Lennox 10 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Clear vents and check hoses
Clear the outdoor intake/exhaust pipes and check the pressure hoses for water, kinks, or cracks. - Call a technician
An intermittent pressure fault needs draft-pressure measurements over time to catch the failing part.
When to call a technician
- The lockout repeats
- The inducer sounds labored or changes speed on its own
Frequently asked questions
Why five times?
The control tolerates a few momentary dropouts (gusts of wind can do it), but five in one call means something is genuinely wrong.
Will it reset itself?
Watchguard lockouts clear after the waiting period or a power cycle — but the underlying intermittent fault will bring the code back.
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.