HVAC Code Lookup

Lennox Furnace Code 12

Safety faultShown as: 12 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats

The control is sensing flame when the gas valve is not energized. Either flame is present when it should not be, or the flame-sensing circuit is reading falsely.

Manual wording: “12 - Flame Sensed without gas valve energized”

Documented for: ML180UHE, ML193UHE and other Lennox furnaces using SureLight integrated control 103217-03 or 107163-01

Likely causes, most common first

  1. Gas valve leaking through and allowing gas to burn when it should be closed
  2. Flame sense circuit or control board fault reading a false flame signal

What you can safely check yourself

  • Turn the furnace power switch off at the switch beside the furnace

When to call a technician

The manual's own instruction for a false flame condition is to shut off the gas and check the gas valve for a leak. Turn the furnace off and call a technician. If you smell gas, leave the house first and call the gas utility from outside.

A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Other Lennox codes