HVAC Code Lookup

Lennox Furnace Code 5

Soft lockoutShown as: 5 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats

Watchguard lockout. The burners failed to light after the control used up its maximum number of ignition retries or recycles, so the furnace has locked out.

Manual wording: “5 - Watchguard - burner failed to light, exceeded maximum number of retries or recycles.”

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, or gas pressure too low to light the burners
  2. Failed hot surface ignitor
  3. Flame sensor unable to prove the flame after repeated lights
  4. Vent or combustion air restriction cutting the ignition sequence short

What you can safely check yourself

  • Check that other gas appliances in the house are working
  • Check the outdoor intake and exhaust pipes for snow, ice or debris blockage
  • Check that the furnace door is fully seated
  • Turn the furnace power switch off for 30 seconds and back on once to clear the lockout, one time only

When to call a technician

A repeated Watchguard lockout means the furnace has already tried and failed to light several times. Stop resetting it and have a technician find out why it will not light.

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

Sources

Other Lennox codes