HVAC Code Lookup

Lennox Furnace Code 3

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

The burners failed to light during a heat call, or a flame that was lit went out before the heat demand was satisfied.

Manual wording: “3 - Burner failed to light, or lost flame during heat demand”

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

Likely causes, most common first

  1. No gas supply to the furnace, or the manual gas valve is closed
  2. Dirty or failed flame sensor so the control cannot prove the flame
  3. Failed or cracked hot surface ignitor
  4. Condensate or vent restriction interrupting the burner cycle

What you can safely check yourself

  • Check that other gas appliances in the house (range, water heater) are working, which tells you whether gas is being delivered
  • Check that the furnace door is fully seated, since the door switch must be closed for the furnace to fire
  • Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
  • Turn the furnace power switch off for 30 seconds and back on once

When to call a technician

If the code returns, the ignitor, flame sensor, gas valve or gas pressure needs to be tested. Do not open the burner compartment or touch the gas valve yourself.

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

Sources

Other Lennox codes