Lennox Furnace Error Codes: 13, 23, 24, 25, 27, 28, 34 — Diagnosis & Repair

Common Symptoms

• Furnace not heating or short-cycling
• LED on the control board blinking in a repeated pattern
• No ignition or furnace locking out after multiple attempts
• Blower running but no heat produced
• Furnace starting then shutting off within seconds

Most Likely Causes

1. 1

   Code 13 — Limit Circuit Lockout

   The furnace has overheated and locked out after multiple high-limit trips. Causes: severely clogged air filter, blocked return air, closed supply registers, or a failed blower motor. The furnace will not restart until manually reset after the heat exchanger cools.

2. 2

   Code 23 — Pressure Switch Stuck Closed

   The inducer pressure switch is reading closed at startup when it should be open. Causes: stuck or defective pressure switch, a pinched or waterlogged pressure hose.

3. 3

   Code 24 — Pressure Switch Stuck Open

   The pressure switch did not close after the inducer reached operating speed. Causes: failed inducer motor not building suction, a cracked/blocked pressure hose, a stuck-open pressure switch, or a blocked flue.

4. 4

   Code 25 — Ignition Failure (Recycle)

   The furnace attempted ignition but no flame was sensed. The furnace will retry several times before locking out. Causes: a failed hot surface igniter, dirty flame sensor, weak gas pressure, or faulty gas valve.

5. 5

   Code 28 — Low Flame Signal

   The flame sensor is detecting an unusually low flame signal. Usually caused by an oxidized flame sensor that needs cleaning, or low gas pressure at the manifold.

6. 6

   Code 34 — Ignition Proving Failure (Lockout)

   The furnace made multiple ignition attempts and could not prove a stable flame. The system has locked out. Requires a manual reset (power cycle) after diagnosing the ignition system.

Quick DIY Checks

1. 1Check the air filter immediately for codes 13 and 24 — a clogged filter is the #1 cause of limit lockouts and reduced airflow. Replace with a fresh filter matching your furnace's size and MERV rating.
2. 2For code 25/34 (ignition failure): Locate the flame sensor — a thin metal rod (about 2 inches long) mounted near the burner. Remove it (one screw) and clean the rod with fine steel wool or a dollar bill folded several times. Oxidation on the sensor causes false 'no flame' detection. Reinstall and try a restart.
3. 3For code 24 (pressure switch stuck open): Inspect the rubber pressure hose connecting the inducer to the pressure switch for cracks, kinks, or water condensate. Blow through the hose to clear any blockage. Also check the furnace flue pipe for ice blockages (common in winter).

1. 4For code 13 (limit lockout): After replacing the filter and allowing the furnace to cool, reset power by turning the furnace off at the switch and back on. Verify all supply and return registers in the home are open. Check the blower wheel for dust buildup.
2. 5For code 28 (low flame): Clean the flame sensor as above. If the signal remains low after cleaning, check the gas manifold pressure — this requires a manometer and is best left to a certified HVAC technician.

Repair vs Replace

Recommended Tools & Parts

• Lennox Furnace Flame Sensor

  Replacement flame sensor for Lennox furnaces. Fixes codes 25, 28, and 34 when cleaning doesn't restore a proper flame signal.

  $15–$30

  Buy on Amazon →

• Lennox Hot Surface Igniter

  Replacement hot surface igniter for Lennox furnaces. Required when igniter glows but flame doesn't ignite, or igniter doesn't glow at all.

  $30–$65

  Buy on Amazon →

• Lennox Furnace Pressure Switch

  Replacement inducer pressure switch for Lennox furnaces. Fixes codes 23 and 24 when the hose and inducer are confirmed good.

  $20–$50

  Buy on Amazon →

• High-MERV Furnace Air Filter

  Replacement air filter for Lennox furnaces. Replace every 1–3 months. A clogged filter is the #1 cause of furnace lockout codes.

  $10–$30

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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