Home Fault Codes

Lennox Furnace 1 Flash: Meaning and Fix

Applies to: Lennox ML180UHE furnaces with integrated control 107163-01 (red LED flash codes).

1 Flash
How 1 Flash appears on a Lennox furnace display.

Also shown as: 1 flash, one flash

Reading the furnace display

A furnace reports faults on a small status LED on the control board, seen through the sight glass in the blower door, or on a two-digit display. Watch a full cycle of the pattern and count the flashes, including the pause between groups, before you decide what you saw. The code chart is on the label inside the blower door.

This page lists the fault codes this brand documents, with likely causes and a short repair path. The codes apply to model numbers such as ML180UHE; check the label on your unit to confirm yours.

What causes 1 Flash

How to fix it

If you smell gas, leave the building and call your gas utility from outside; do not touch switches or the furnace. Switch the furnace power off before you open the blower door or touch the igniter or flame sensor, and call a licensed HVAC technician if you are not sure.

The code meaning comes from the manufacturer document listed under Sources. The causes and steps below are general troubleshooting guidance, not the manufacturer's service procedure.

  1. Switch the furnace power off at the breaker before touching any wiring.
  2. Do not defeat the polarity check.
  3. A licensed electrician or technician should check hot and neutral at the furnace.
  4. Check that wall switch and junction box connections are tight, with power off.
  5. Restore power and run a heating cycle. If the code returns, call a licensed HVAC technician and give them the code, the board label and the furnace model number.

Questions

What does 1 Flash mean on a Lennox furnace?

On a Lennox ML180UHE control board (107163-01), 1 red LED flash mean: reverse line voltage polarity or phasing of 120V power.

Can I reset the furnace myself?

The control can be reset by switching power off at the furnace switch or breaker for about 30 seconds, which clears some lockouts. If the same code returns, the fault is still present and the control will shut the furnace down again. Never bypass a safety switch to keep it running.

When should I call a technician?

Call a licensed HVAC technician for anything involving gas, the pressure switch, the limit or rollout switches, the igniter, the flame sensor or a control board replacement, or if the code keeps coming back after the simple checks.

Other brands with code 1 Flash

The same code can mean something different on another maker's appliance. Check the brand before you act.

Chamberlain garage door openerSafety sensor wire broken or disconnectedCraftsman garage door openerSafety sensor wire broken or disconnectedGoodman furnaceFlame sensed when no flame should be presentLiftMaster garage door openerSafety sensor wire broken or disconnected

Other Lennox furnace codes

2 FlashesImproper earth ground3 FlashesBurner failed to light, or lost flame during heat demand4 FlashesLow flame signal, check flame sensor5 FlashesWatchguard: burner failed to light, exceeded maximum retries or recycles (1 hour lockout)7 FlashesPrimary or secondary limit switch open, or Watchguard: limit open longer than 3 minutes8 FlashesRoll-out switch open

Sources

Part numbers are OEM references; confirm fit by your model number before ordering. Meanings compiled from manufacturer and parts-retailer documentation; see sources below. Unplug the appliance before opening it.

Applies to models including ML180UHE.