HVAC Code Lookup

Lennox Furnace Code 9

WarningShown as: 9 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats

The pressure prove switch did not close when the combustion air inducer started, or it opened part way through the heat demand.

Manual wording: “9 - Pressure Switch failed to close or opened 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. Blocked or restricted intake or exhaust vent pipe
  2. Plugged condensate drain or trap backing water into the pressure switch tubing
  3. Cracked, kinked or disconnected pressure switch hose
  4. Weak or failed combustion air inducer

What you can safely check yourself

  • Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
  • Check the condensate drain line and trap for a clog or standing water
  • Check that the furnace door is fully seated
  • Turn the furnace power switch off for 30 seconds and back on once

When to call a technician

If the vent is clear and the drain is clear and the code stays, the pressure switch, its tubing or the inducer needs testing. Never jumper a pressure 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