Lennox Furnace 1 error code
One flash means the line-voltage wiring is reversed in polarity or phasing — hot and neutral are swapped, or the furnace is not properly phased to ground.
Also shown as: 1 flash, one flash, single flash, fault code 1, reverse polarity
Last updated: September 30, 2026
What causes the 1 error?
- Hot and neutral wires swapped at the furnace or breaker panel
- Wiring error during installation or service
- Floating or improperly bonded neutral
How to fix Lennox 1 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Do not attempt a DIY fix
Correcting polarity requires working inside the breaker panel and the furnace junction box with live-voltage testing.Safety warning: Line-voltage wiring mistakes can energize the furnace chassis. Only a licensed electrician or HVAC technician should correct polarity — turn the furnace breaker off and call a professional. - Call a professional
An electrician or HVAC technician will verify hot, neutral, and ground with a meter and correct the wiring.
When to call a technician
- Immediately — this is an electrical wiring fault
- If the furnace was recently installed or serviced
Frequently asked questions
Can reversed polarity damage the furnace?
Yes — it can prevent proper flame sensing and, in some cases, energize parts of the chassis. It must be corrected.
Why would this appear suddenly?
It usually follows electrical work: a new breaker, a moved circuit, or service on the furnace's power wiring.
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.