Carrier Furnace Blink Codes: 13, 14, 24, 31, 33 — How to Read & Fix

Common Symptoms

• LED(s) on the control board blinking in a specific counted pattern
• Furnace not heating or short-cycling
• Furnace attempts ignition but locks out after a few tries
• Blower running continuously but no heat output
• Secondary LED on board lit or blinking alongside the status LED

Most Likely Causes

1. 1

   Code 13 — Limit Device Lockout

   The furnace tripped the high-limit switch more than once in a single demand cycle and entered limit device lockout. The furnace will not attempt ignition again until manually reset. Root cause: severely clogged air filter, blocked return air duct, closed registers, or a failed blower motor running below proper speed. On Carrier WeatherMaker and Infinity series, code 13 lockout requires turning the thermostat off and back on, or cycling the breaker, to resume normal operation. Always replace the filter before resetting.

2. 2

   Code 14 — Ignition Lockout

   The furnace attempted ignition the maximum number of times (typically 3 attempts on Carrier) without proving a flame and locked out. The board will not attempt ignition again until manually reset. Causes: a failed hot surface igniter, an oxidized flame sensor unable to pass the flame rectification current, inadequate gas pressure, or a faulty gas valve coil. On Carrier boards with separate LED indicators, code 14 shows as 1 flash on the first LED and 4 flashes on the second LED.

3. 3

   Code 24 — Secondary Voltage Fuse Blown

   The 3-amp automotive-style fuse on the control board has blown. This fuse protects the secondary (24VAC) control circuit. Causes: a shorted thermostat wire (staple through the wire, wire pinched in a door), a shorted accessory on the low-voltage circuit (humidifier, zone board, EAC), or a failing transformer overloading the secondary. Do not replace the fuse without finding the short — it will blow again immediately. Carrier boards with code 24 will show 2 flashes on LED 1 and 4 flashes on LED 2.

4. 4

   Code 31 — Pressure Switch Stuck Open

   The draft inducer ran at startup but the pressure switch did not close within the expected time. Causes: an inducer motor spinning but not building sufficient draft (worn bearings, cracked wheel, dirty housing), a blocked or cracked pressure hose, a blocked flue or condensate drain on 90+ efficiency models, or a defective pressure switch. Unlike code 32 (switch stuck closed), code 31 means the switch never activated.

5. 5

   Code 33 — Limit Circuit Fault / Rollout Switch Lockout

   The limit circuit has opened during operation and caused lockout. This is similar to code 13 but specifically implicates the rollout switch or a combination of limit and rollout trips. A rollout switch trips when flames extend outside the normal combustion zone — indicating a blocked heat exchanger, a cracked heat exchanger allowing backflow, or a blocked flue that forces combustion gases backward. Code 33 is a serious fault: if the rollout switch cannot be reset manually (red button on the switch), the heat exchanger may be cracked.

Quick DIY Checks

1. 1How to read Carrier dual-LED blink codes: Carrier control boards from the mid-2000s onward use a two-LED diagnostic system. LED 1 (top or labeled 'ALERT') flashes the tens digit. LED 2 (bottom or labeled 'STATUS') flashes the units digit. Example: LED 1 flashes 3 times and LED 2 flashes 3 times = code 33. Count both sequences before looking up the code. Older single-LED Carrier boards (pre-2000s, round boards) work like Goodman — the LED flash count directly equals the code. If the status LED is showing a rapid alternating blink with no discrete count, the board is in continuous retry mode. On Carrier Infinity and Performance series, you can also read fault codes from the thermostat — navigate to the diagnostics menu to see stored fault history.
2. 2For code 13 or 33 (limit or rollout lockout): replace the air filter before anything else — a severely clogged filter is the primary cause of limit lockouts on Carrier furnaces. After replacing the filter, locate the manual rollout reset button (red disc-shaped button on the rollout switch in the burner compartment) and press it to reset the rollout switch if it has tripped. On code 33, inspect the burner area with a flashlight — any evidence of scorch marks outside the normal burner flame path, or heat damage to the rollout switch or wiring, indicates flames were rolling out and the heat exchanger or flue needs professional inspection.
3. 3For code 14 (ignition lockout): turn off the furnace at the breaker. Locate and test the hot surface igniter — remove it from the burner assembly (one screw, then unplug the connector) and test resistance with a multimeter set to ohms. Silicon nitride igniters (most common on Carrier since the late 1990s) read 40–90 ohms when functional. OL (open circuit) means the element has cracked and needs replacement. Next, locate the flame sensor rod (a thin metal rod in the burner, single wire connector with ceramic insulator) and clean the metal tip with 120-grit emery cloth — do not buff the ceramic. Reinstall both components, restore power, and attempt a heat cycle. Monitor through the sight glass to see whether the igniter glows and the burner lights.

1. 4For code 24 (secondary voltage fuse blown): locate the 3-amp mini-blade automotive fuse on the control board (check the board label for its location — typically labeled 'SEC FUSE' or '3A'). Test it with a multimeter in continuity mode — no beep confirms it is blown. Before installing a replacement, identify the short. Disconnect the wire at the Y (cooling), G (fan), W (heat), and accessories terminals on the board one at a time and measure resistance between each wire and ground. Any wire reading less than 10 ohms to ground has a short. Check the thermostat wire run for staples driven through the cable, wire pinched in a door frame, or bare wire contacting the furnace chassis. Replace the fuse only after the short is corrected.
2. 5For code 31 (pressure switch stuck open): listen to the inducer motor during a heat call. On Carrier 80% furnaces, the inducer should spin up within seconds of a heat demand and you should hear a distinct hum. On 90+ efficiency furnaces, you should hear both an inducer and a secondary condensate drain operating. If the inducer sounds weak or noisy (grinding, rattling), the motor or wheel may be failing. Inspect the pressure hose from the inducer housing to the pressure switch — look for kinks, condensate blockage, or cracks. On 90+ efficiency Carrier condensing furnaces, also check the secondary heat exchanger condensate drain line for blockage — a clogged drain traps condensate and starves the inducer of suction. Blow through the hose to clear it. Test the pressure switch by disconnecting the hose from the switch nipple and applying gentle suction — continuity should click in. No click means the switch has failed.

Repair vs Replace

Recommended Tools & Parts

• Carrier Bryant Hot Surface Igniter (Silicon Nitride)

  Replacement hot surface igniter for Carrier and Bryant furnaces — fixes code 14 ignition lockout. Common Carrier part numbers include LH33ZS004, LH33ZS002, and KGAPK0101AAA. Verify voltage (120V or 24V model specific) before ordering.

  $30–$80

  Buy on Amazon →

• Carrier Flame Sensor Rod

  OEM-style flame sensor replacement for Carrier and Bryant gas furnaces. Fixes repeated code 14 ignition lockout after cleaning fails. Match the mounting bracket style to your existing sensor.

  $15–$40

  Buy on Amazon →

• Carrier Draft Inducer Pressure Switch

  Replacement pressure switch for Carrier and Bryant furnaces — fixes code 31 (stuck open) and code 32 (stuck closed) when hose is verified clear. Match the wye-number (WC rating) on your existing switch.

  $25–$60

  Buy on Amazon →

• 3-Amp Mini-Blade Automotive Fuse (10-Pack)

  Replacement 3-amp fuse for Carrier furnace control board — fixes code 24 secondary voltage fuse blown. Always identify and fix the short before replacing the fuse.

  $5–$12

  Buy on Amazon →

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

Related Repairs