Lennox furnace codes
A furnace reports faults on a small status LED on the control board, seen through the sight glass in the blower door, or on a two-digit display. Watch a full cycle of the pattern and count the flashes, including the pause between groups, before you decide what you saw. The code chart is on the label inside the blower door.
Models covered: ML180UHE.
All 12 codes
1 FlashReverse line voltage polarity or phasing of 120V power (pro repair)2 FlashesImproper earth ground (pro repair)3 FlashesBurner failed to light, or lost flame during heat demand (pro repair)4 FlashesLow flame signal, check flame sensor (pro repair)5 FlashesWatchguard: burner failed to light, exceeded maximum retries or recycles (1 hour lockout) (pro repair)7 FlashesPrimary or secondary limit switch open, or Watchguard: limit open longer than 3 minutes (pro repair)8 FlashesRoll-out switch open (pro repair)9 FlashesPressure switch failed to close or opened during heat demand, inducer on (pro repair)10 FlashesWatchguard: pressure switch opened 5 times during a single heating demand (pro repair)11 FlashesPressure switch stuck closed before the combustion air inducer starts (pro repair)12 FlashesFlame sensed without gas valve energized (pro repair)13 FlashesLow line voltage (pro repair)
About Lennox codes
This page lists the fault codes this brand documents, with likely causes and a short repair path. The codes apply to model numbers such as ML180UHE; check the label on your unit to confirm yours.