Lennox Furnace 12 Flashes: Meaning and Fix

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.

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. If you smell gas or can see flame while the thermostat is not calling for heat, switch the furnace off, leave the building and call your gas utility from outside.
2. Do not keep running the furnace.
3. A technician should check the gas valve and flame sense wiring.
4. Do not attempt gas valve repair yourself.
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.

What does 12 Flashes mean on a Lennox furnace?

On a Lennox ML180UHE control board (107163-01), 12 red LED flashes mean: flame sensed without gas valve energized.

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.

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.

• https://www.lennox.com/dA/9b51323cc2/507103-04a.pdf