Lennox Furnace Code 11
WarningShown as: 11 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats
The pressure switch contacts were already closed before the combustion air inducer was turned on. The control will not light the burners because it cannot prove the vent is actually drawing.
Manual wording: “11 - Pressure Switch stuck closed prior to activation of combustion air inducer”
Documented for: ML180UHE, ML193UHE and other Lennox furnaces using SureLight integrated control 103217-03 or 107163-01
Likely causes, most common first
- Pressure switch stuck closed
- Pressure switch tubing routed or connected incorrectly
- Shorted pressure switch wiring
What you can safely check yourself
- Turn the furnace power switch off for 30 seconds and back on once
When to call a technician
A pressure switch that reads closed at rest is a failed safety device and has to be replaced by a technician. It must never be jumpered out.
A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation Instructions ML180UHE (507103-04, 04/2024) - Integrated Control 103217-03 and 107163-01 RED LED Flash Code tables (Lennox Industries Inc., ML180UHE series 80% AFUE upflow/horizontal gas furnace) · checked 2026-08-25
- Installation Instructions ML193UHE (507863-02, 06/2020) - Table 10, RED LED Flash Code / Diagnostic Codes / Status of Furnace (Lennox Industries Inc., ML193UHE series 93% AFUE upflow/horizontal gas furnace) · checked 2026-08-25