HVAC Code Lookup

Lennox Furnace Code 10

Soft lockoutShown as: 10 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats

Watchguard lockout because the pressure switch opened five times during a single heating demand. The furnace has given up on that heat call.

Manual wording: “10 - Watchguard - Pressure Switch opened 5 times during one 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. Partially blocked intake or exhaust vent
  2. Plugged condensate drain or trap
  3. Marginal or failing combustion air inducer
  4. Loose or wet pressure switch tubing

What you can safely check yourself

  • Check the outdoor intake and exhaust pipes for snow, ice or debris
  • Check the condensate drain line and trap for a clog
  • Turn the furnace power switch off for 30 seconds and back on once, one time only

When to call a technician

Five pressure switch trips in one cycle points at a venting, drain or inducer problem a technician needs to measure. Do not bypass the switch.

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

Sources

Other Lennox codes