Carrier AC Error Codes: 12, 23, 24, 33, 34, 44 — What They Mean & How to Fix

Common Symptoms

• AC or furnace not turning on or short-cycling
• LED on the air handler blinking in a specific pattern
• Blower motor not running
• No ignition or igniter glowing but no flame
• System locking out after a few attempts

Most Likely Causes

1. 1

   Code 12 — Blower Motor Fault

   The indoor blower motor is not running at the expected RPM. Causes include a failed capacitor, a seized blower motor, a failed ECM motor module, or a blocked blower wheel due to debris buildup.

2. 2

   Code 23 — Pressure Switch Stuck Closed

   The draft inducer pressure switch is reading closed when it should be open. A stuck pressure switch, a waterlogged pressure tube, or a failed pressure switch keeps the system from completing the startup sequence.

3. 3

   Code 24 — Secondary Limit / High Limit Tripped

   The heat exchanger is overheating. Most common cause is a dirty air filter restricting airflow. Can also be caused by a blocked return air duct or a failed blower motor.

4. 4

   Code 33 — Limit Circuit Fault

   A high-temperature limit switch has opened, indicating overheating. Common causes: dirty air filter, blocked registers, or a failed blower motor not moving sufficient air over the heat exchanger.

5. 5

   Code 34 — Ignition Failure

   The furnace attempted to light but no flame was detected. Common causes: a failed hot surface igniter, a dirty or cracked flame sensor, a faulty gas valve, or no gas supply.

6. 6

   Code 44 — Inducer Motor Fault

   The draft inducer motor is not running or not reaching speed. The inducer motor removes combustion gases from the heat exchanger — if it fails, the furnace locks out for safety.

Quick DIY Checks

1. 1Check and replace the air filter first — a clogged filter causes codes 24 and 33 (overheating) and can contribute to blower problems. Replace with a fresh filter matching the unit's MERV rating.
2. 2For code 34 (ignition failure): Locate the flame sensor — a thin metal rod in the burner area. Use fine steel wool or a dollar bill to lightly polish the sensor rod to remove oxidation, which causes false 'no flame' readings. Reconnect and try a start cycle.
3. 3For code 12 (blower fault): Locate the blower motor capacitor (a cylindrical or oval component on the blower motor). A failed capacitor looks visually bulged or has a loose terminal. Test with a multimeter on capacitance mode — replace if outside the rated value (usually printed on the capacitor label).

1. 4For code 23 (pressure switch): Disconnect the rubber pressure tube from the pressure switch and blow through it to clear any condensate blockage. Reconnect and test. If the code persists, the pressure switch may need replacement.
2. 5For codes 33/24: After replacing the filter, check all supply and return registers — make sure at least 80% are open. Verify that the blower wheel isn't packed with dust by looking through the blower access panel.
3. 6For code 44 (inducer motor): Listen for the inducer motor attempting to run at startup (it should spin up first before ignition). If you hear nothing, the motor or its capacitor has failed. This typically requires a professional replacement.

Repair vs Replace

Recommended Tools & Parts

• Furnace Run Capacitor

  Replacement run capacitor for blower motors and inducer motors. Check the microfarad (MFD) rating on your existing capacitor label before ordering.

  $10–$30

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• Universal Hot Surface Igniter

  Replacement hot surface igniter (silicon nitride type). Fixes code 34 ignition failures. Check your carrier model for correct wattage.

  $25–$55

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• Furnace Flame Sensor

  Replacement flame sensor rod for Carrier and Bryant furnaces. Fixes recurring code 34 after cleaning fails.

  $10–$25

  Buy on Amazon →

• Draft Inducer Pressure Switch

  Replacement pressure switch for Carrier furnaces. Fixes code 23 when the tube is clear but the switch still reads incorrectly.

  $20–$50

  Buy on Amazon →

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

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