Lennox Furnace 13 error code
Thirteen flashes mean the line voltage reaching the furnace is too low for reliable operation.
Also shown as: 13 flashes, thirteen flashes, fault code 13, low line voltage
Last updated: September 30, 2026
What causes the 13 error?
- Utility brownout or sag
- Loose wiring connection
- Undersized or overloaded circuit
- Failing transformer
How to fix Lennox 13 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Check the breaker
Make sure the furnace breaker is fully on — a half-tripped breaker can cause low voltage. - Look for wider power issues
If lights dim or other appliances act up, the problem is likely the utility supply or house wiring — call an electrician. - Call a technician
Persistent low voltage at the furnace alone needs professional electrical diagnosis.Safety warning: Do not open the breaker panel or furnace junction box yourself. Low-voltage symptoms can mask dangerous loose connections that arc and overheat.
When to call a technician
- The code persists
- You notice dimming lights or warm/burning smells at outlets or the panel
Frequently asked questions
Can low voltage damage the furnace?
Yes — motors and controls can overheat or behave erratically on low voltage, which is why the board flags it.
Will the furnace run anyway?
It may try, but operation will be unreliable. Fix the voltage problem first.
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.