Carrier Furnace Error Codes: Complete Troubleshooting Guide [2026]

Understanding Carrier Furnace Error Codes

When your Carrier furnace stops working, the control board communicates the problem through a system of LED flash codes. These diagnostic lights are your furnace’s way of telling you exactly what went wrong — if you know how to read them.

At Furnace King Home Services, we’ve been servicing Carrier furnaces across Mississauga, Etobicoke, Brampton, and Oakville for over 40 years. In this guide, I’ll walk you through every Carrier furnace error code, explain what each one means, and help you determine whether you can troubleshoot the issue yourself or need to call a professional.

How to Read Carrier Furnace LED Flash Codes

Carrier furnaces use a two-digit diagnostic code system displayed through LED lights on the integrated furnace control (IFC) board. Here’s how to find and read them:

Step 1: Locate the Diagnostic LED

The control board is located in the lower compartment of your furnace. Most Carrier models have a small, clear plastic sight glass (inspection port) in the centre of the lower furnace access panel. You can observe the LED flashing through this window without removing the panel.

Step 2: Understand the Two-Digit System

Carrier uses two LEDs — typically a yellow (amber) LED and a green LED — to communicate status codes:

• The first digit is the number of times the yellow LED flashes
• The second digit is the number of times the green LED flashes
• For example, 3 yellow flashes followed by 2 green flashes = Status Code 32

Step 3: Normal Operation Indicators

• Both LEDs flash slowly and simultaneously: Normal standby — no call for heat
• Both LEDs flash rapidly and simultaneously: Normal call for heat in progress
• Both LEDs on steady: System is in test/diagnostic mode

Important: The LED sequence repeats in a continuous loop. Watch for at least two full cycles to confirm the code. The last five fault codes are stored in memory, so you may see the most recent code displayed even after the furnace has been reset.

Complete Carrier Furnace Error Code Chart

Below is the complete list of Carrier furnace status codes used on modern IFC control boards. These codes apply to popular models including the Carrier 59SC, 59TP, 58MVC, 58MVP, 59MN7, and similar series.

Normal Operation Codes

Code	LED Pattern	Meaning	Action Required
11	1 Yellow, 1 Green	No previous fault codes stored	None — system is operating normally
12	1 Yellow, 2 Green	Blower ON — system energised at power-up with thermostat calling for heat	None — blower runs for 90 seconds, then normal heating cycle begins

Ignition & Flame Fault Codes

Code	LED Pattern	Meaning	Common Causes	Action
13	1 Yellow, 3 Green	Limit circuit lockout — furnace has locked out after repeated limit switch trips	Dirty air filter restricting airflow; blocked return air vents; failed blower motor; faulty limit switch	Replace air filter and check all vents are open. If code persists, call a professional
14	1 Yellow, 4 Green	Ignition lockout — furnace failed to ignite in 4 attempts	Faulty hot surface igniter; dirty flame sensor; gas supply interruption; defective gas valve; loose ground wire	Check gas supply is on. Call a professional — this involves gas components
21	2 Yellow, 1 Green	Gas heating lockout — gas valve energised but no flame detected	Gas supply off; faulty gas valve; defective flame sensor; igniter failure	Verify gas supply. Call a professional immediately
34	3 Yellow, 4 Green	Ignition proving failure — flame detected then lost during blower on-delay	Dirty flame sensor; cracked heat exchanger; low gas pressure; intermittent gas valve	Call a professional — possible heat exchanger issue

Pressure Switch Fault Codes

Code	LED Pattern	Meaning	Common Causes	Action
22	2 Yellow, 2 Green	Abnormal flame-proving signal — flame detected when no flame should be present	Leaky gas valve; residual flame; faulty flame sensor	Shut off furnace immediately and call a professional — potential gas leak hazard
23	2 Yellow, 3 Green	Pressure switch stuck closed — switch remains closed when inducer motor is off	Pressure switch hose blocked or disconnected; faulty pressure switch; short in wiring	Check pressure switch hose for blockages. Call a professional if code persists
24	2 Yellow, 4 Green	Secondary voltage fuse is open — no 24V power to thermostat circuit	Short circuit in thermostat wiring; faulty thermostat; shorted gas valve coil	Check thermostat wiring for damage. Call a professional to replace fuse and diagnose short

Limit & Safety Switch Codes

Code	LED Pattern	Meaning	Common Causes	Action
31	3 Yellow, 1 Green	High-pressure switch fault — switch closed at inappropriate time or fails to close	Blocked flue or intake pipe; faulty pressure switch; inducer motor failure	Check flue and intake pipes for blockages (ice, bird nests, debris). Call a professional
32	3 Yellow, 2 Green	Low-pressure switch fault — switch open during operation	Blocked condensate drain; cracked pressure switch hose; weak inducer motor	Check condensate drain for blockages. Call a professional
33	3 Yellow, 3 Green	Limit circuit fault — limit switch or flame rollout switch is open	Overheating due to restricted airflow; dirty filter; blocked vents; failed blower; flame rollout condition	Replace air filter. Check all supply and return vents. Call a professional — may indicate heat exchanger problem

Blower & Inducer Motor Codes

Code	LED Pattern	Meaning	Common Causes	Action
41	4 Yellow, 1 Green	Blower outside valid speed range — blower motor not running at calculated RPM	Dirty blower wheel; failing blower motor; incorrect blower speed settings; wiring issue	Call a professional — blower motor may need service or replacement
42	4 Yellow, 2 Green	Inducer outside valid speed range — inducer motor not running at calculated RPM	Failing inducer motor; blocked flue; dirty inducer wheel; wiring fault	Call a professional — inducer motor diagnostics required
43	4 Yellow, 3 Green	Pressure switch calibration fault — pressure switch readings out of range during purge cycle	Faulty pressure switch; blocked vent; incorrect switch installed	Call a professional — requires recalibration or switch replacement
44	4 Yellow, 4 Green	Blower calibration fault — calculated blower speed below 250 RPM or above 1,300 RPM	Incorrect blower motor; dirty blower wheel; wiring issue; control board fault	Call a professional — motor or control board replacement likely needed

Additional LED Indicators

LED State	Meaning	Action
No LEDs on	No 120V AC power to control board	Check circuit breaker and furnace power switch. If both are on, call a professional
Both LEDs on continuously	Control is in test/setup mode	Turn off and restore power to exit test mode
Yellow LED only — rapid flash	Flame detected without call for heat	Shut off furnace and call a professional immediately — potential gas valve leak
Green LED only — rapid flash	Line voltage (115V AC) polarity reversed or poor grounding	Have an electrician check the electrical supply to the furnace

Most Common Carrier Furnace Problems

After 40 years of servicing Carrier furnaces in the GTA, these are the five error codes we see most often at Furnace King:

1. Code 14 — Ignition Lockout

This is the single most common Carrier furnace error we encounter. Your furnace tried to ignite up to four times and failed each time, so it locked itself out as a safety measure.

Why it happens: The most frequent culprit is a dirty flame sensor — a small metal rod that sits in the burner flame and confirms ignition. Over time, carbon buildup insulates the sensor so it can’t detect the flame, even though the burner is lit. The furnace thinks ignition failed and shuts down the gas.

What you can do: Code 14 resets automatically after 3 hours, or you can manually reset by turning the furnace off at the power switch for 30 seconds. If the code keeps returning, the flame sensor likely needs cleaning or replacement — a quick, affordable repair for a qualified technician.

2. Code 33 — Limit Circuit Fault

The limit switch is a critical safety device that shuts your furnace down when it overheats. When Code 33 appears, it means the furnace reached an unsafe temperature and the limit switch opened to protect the heat exchanger.

Why it happens: In most cases, the root cause is restricted airflow. A clogged air filter is the number one culprit, followed by closed or blocked supply registers, a dirty blower wheel, or a failing blower motor. In Canadian winters, we also see this when homeowners close too many vents trying to redirect heat.

What you can do: Check and replace your furnace filter immediately. Make sure all supply and return vents throughout your home are open and unblocked by furniture or curtains. If the code keeps coming back with a clean filter, your blower motor may need attention.

3. Code 31 — Pressure Switch Fault

The pressure switch verifies that the inducer motor is pulling combustion gases through the heat exchanger and up the flue. Code 31 means this safety check has failed.

Why it happens: During harsh Ontario winters, the most common cause is ice or snow blocking the exhaust vent or fresh air intake pipe on the exterior wall of your home. Birds’ nests, leaves, and other debris can also cause blockages. Less commonly, the pressure switch hose may be cracked, disconnected, or filled with condensate.

What you can do: Go outside and check your furnace exhaust and intake pipes. Clear any ice, snow, or debris. If the pipes are clear and the code persists, call a professional.

4. Code 23 — Pressure Switch Stuck Closed

This code means the pressure switch is reporting a “closed” position even when the inducer motor is off — the opposite of what should happen.

Why it happens: The pressure switch hose may be blocked with condensate (water), creating a false reading. A faulty pressure switch can also get stuck mechanically. In some cases, a short in the wiring between the switch and control board causes this code.

What you can do: This typically requires professional diagnosis. The technician will check the hose, test the switch with a manometer, and inspect wiring.

5. Code 42 — Inducer Motor Fault

The inducer motor creates the draft needed to safely vent combustion gases. Code 42 indicates it’s not running at the correct speed.

Why it happens: A failing inducer motor bearing is the most common cause, especially on furnaces over 10 years old. A blocked flue or a dirty inducer wheel can also cause the motor to run outside its expected RPM range.

What you can do: If you hear an unusual humming, grinding, or vibrating noise from the furnace, the inducer motor bearings are likely worn. This requires professional replacement — do not ignore this code, as a non-functioning inducer motor can lead to dangerous combustion gas buildup.

When to Call a Professional vs. DIY Troubleshooting

Your safety comes first — always. Here’s a clear guide on what you can safely check yourself and when you need a licensed HVAC technician.

Safe to Check Yourself

• Replace the furnace filter — a dirty filter causes more error codes than any other single issue
• Check vents and registers — make sure all supply and return vents are open and unblocked
• Inspect exterior pipes — clear ice, snow, or debris from the exhaust and intake pipes outside your home
• Verify gas supply — confirm the gas valve near your furnace and your gas metre are both in the “on” position
• Check the thermostat — ensure it’s set to “Heat” mode and the set temperature is above room temperature
• Reset the furnace — turn the power switch off, wait 30 seconds, and turn it back on

Call a Professional Immediately

• You smell gas or suspect a gas leak — leave the house and call your gas utility’s emergency line
• The furnace displays Code 22 (flame without call for heat) — potential gas valve leak
• Carbon monoxide detectors are alarming — evacuate and call 911
• The furnace keeps locking out on Code 14 after multiple resets
• You see Code 33 (limit circuit fault) repeatedly — possible heat exchanger crack
• Any code related to the gas valve, flame rollout, or inducer motor
• The furnace makes unusual banging, popping, or grinding noises

How to Reset Your Carrier Furnace

After identifying and addressing the issue behind an error code, you may need to reset your Carrier furnace:

1. Turn off the power switch on the side of the furnace (or the circuit breaker)
2. Wait 30 seconds to allow the control board to fully discharge
3. Turn the power back on
4. Observe the LED lights — they should show Code 11 (no faults) or begin a normal startup sequence
5. If the error code returns, do not keep resetting — the underlying problem needs to be fixed

Note: Code 14 (ignition lockout) automatically resets after 3 hours. Codes 13 (limit lockout) and 33 (limit circuit fault) require the furnace to cool down before the limit switch resets. Code 33 with a flame rollout trip requires manually pressing the reset button on the rollout switch, which is located near the burners.

Popular Carrier Furnace Models in Canadian Homes

The error codes listed above apply to most modern Carrier gas furnaces with IFC control boards, including these popular models we frequently service across the GTA:

• Carrier 59SC5 (Comfort Series) — Single-stage, 80% AFUE
• Carrier 59TP6 (Performance Series) — Two-stage, 96% AFUE
• Carrier 59MN7 (Infinity Series) — Variable-speed, modulating, up to 98.5% AFUE
• Carrier 58MVC (Legacy) — Variable-speed, two-stage condensing
• Carrier 58MVP — Variable-speed, two-stage condensing
• Carrier 58STA/58STX — Single-stage, standard efficiency

While the status codes are broadly consistent across these models, always refer to the diagnostic label inside your furnace’s lower access panel for model-specific code definitions.

For more information, visit the ENERGY STAR Heating & Cooling resource guide.

Frequently Asked Questions

What does 2 flashes on a Carrier furnace mean?

Two flashes on a Carrier furnace refer to the first digit of a two-digit status code. The meaning depends on the second digit (the green LED flashes). Code 21 means gas heating lockout (gas valve on but no flame detected). Code 22 means abnormal flame-proving signal (flame detected when it shouldn’t be — a serious safety concern). Code 23 means the pressure switch is stuck closed. Code 24 means the secondary voltage fuse has blown. In all cases, if you see 2 yellow flashes followed by any green flash pattern, it’s best to call a licensed HVAC technician.

How do I read my Carrier furnace error code?

Look through the small sight glass on the lower access panel of your furnace. You’ll see two LEDs — yellow and green. Count the number of yellow flashes (this is the first digit) and then count the green flashes (this is the second digit). For example, 3 yellow flashes + 3 green flashes = Code 33 (limit circuit fault). The pattern repeats continuously, so watch for at least two full cycles to confirm. If both LEDs flash slowly together, your furnace is in normal standby mode.

Why is my Carrier furnace flashing 3 times?

Three flashes of the yellow LED indicate the first digit of your error code is “3.” You need to also count the green LED flashes. Code 31 = high-pressure switch fault (often caused by a blocked exhaust vent). Code 32 = low-pressure switch fault (blocked condensate drain or weak inducer). Code 33 = limit circuit fault (overheating, usually from a dirty filter or blocked vents). Code 34 = ignition proving failure (flame detected then lost). The most common of these is Code 33, which is often resolved by simply replacing a clogged furnace filter.

What does a steady LED light on a Carrier furnace mean?

If both the yellow and green LEDs are on steadily (not flashing), the control board is in test or diagnostic mode. This can happen during initial setup or after a technician has accessed the diagnostic menu. To exit test mode, turn off power to the furnace for 30 seconds and restore it. If no LEDs are on at all, the control board has no power — check your circuit breaker and the furnace’s power switch. A single LED on steadily while the other is off may indicate a board-level issue that requires professional diagnosis.

How do I reset my Carrier furnace after an error code?

To reset your Carrier furnace: (1) Turn off the power switch on the side of the furnace or flip the circuit breaker. (2) Wait at least 30 seconds. (3) Restore power. The control board will restart and should display Code 11 (no faults) if the issue has been resolved. Some codes like Code 14 (ignition lockout) automatically reset after 3 hours. If the flame rollout switch tripped (part of Code 33), you’ll need to press the manual reset button on the rollout switch — it’s a small button on a disc-shaped device near the burners. Important: if the error code returns after resetting, the underlying problem hasn’t been fixed — continuing to reset without repair can cause further damage.

Wondering about replacement costs? Use our HVAC cost estimator to get an instant price range for a new furnace or heat pump system.

Expert Carrier Furnace Service in the GTA

If your Carrier furnace is displaying an error code and you’re not sure what to do, Furnace King Home Services is here to help. Our licensed HVAC technicians have been diagnosing and repairing Carrier furnaces across Mississauga, Etobicoke, Brampton, and Oakville for over 40 years.

We offer same-day diagnosis, upfront pricing, and guaranteed workmanship. Whether it’s a simple flame sensor cleaning or a full control board replacement, we’ll get your heating system back up and running fast.

Call Furnace King at (905) 564-5464 for expert Carrier furnace repair and diagnosis.