Carrier Furnace Blinking Red Light: Codes 13, 33, and 34

Your house is getting cold. You can feel the temperature dropping, and the thermostat is set to 72, but the actual room temperature is reading 64 and falling.

You walk down to the basement or utility room to check the furnace. It is quiet. Too quiet. You look through the small plastic “peep hole” on the bottom door of the unit, and you see it. A red light is pulsing in the darkness.

That light is not just a warning; it is a specific language. Your Carrier furnace is trying to tell you exactly what component has failed.

If you can read that code, you can likely fix the problem yourself for the cost of a new filter or a piece of steel wool. If you ignore it or guess, you could be facing a frozen house or an expensive emergency service call.

At Fixurge, we have diagnosed thousands of Carrier units. The most common codes we see during the winter months are Code 33 (Overheating) and Code 34 (Ignition Failure).

This guide will teach you how to read the unique Carrier “Short and Long” flash system and how to fix the three most common error codes that leave homeowners in the cold.

How to Read Carrier Furnace Blink Codes (The 2-Digit System)

Before we fix anything, we must interpret the signal correctly. This is where most homeowners (and even some rookie technicians) get it wrong.

Carrier furnaces generally do not use a simple “count the flashes” system like other brands. They use a 2-Digit Code System.

The code consists of two numbers: the First Digit (Tens) and the Second Digit (Ones).

The Logic:

1. Short Flashes = The First Digit (10s): The light will flash quickly. Count these rapid flashes.
2. Long Flashes = The Second Digit (1s): The light will pause briefly and then flash slowly. Count these long flashes.

Example: Code 33

You will see: Short-Short-Short (Pause) Long-Long-Long.

• 3 Short Flashes = 30
• 3 Long Flashes = 3
• Total Code = 33

Example: Code 13

You will see: Short (Pause) Long-Long-Long.

• 1 Short Flash = 10
• 3 Long Flashes = 3
• Total Code = 13

Important Note: Do not confuse “Rapid Flashing” (Heartbeat) with an error code. If the light is pulsing steadily and fast without pausing, that is often a “Call for Heat,” meaning the furnace is functioning normally and trying to satisfy the thermostat. An error code always has a distinct pattern or pause.

Code 33: Limit Circuit Fault (The “Dirty Filter” Code)

If you count Code 33 (3 Short, 3 Long), do not panic. This is the most common code we see, and it is usually a maintenance issue, not a broken part.

Technical Definition: Limit Circuit Fault.

Translation: Your furnace overheated.

Inside your furnace, there is a safety component called the High Limit Switch. Its job is to monitor the temperature of the heat exchanger (the metal belly of the furnace). If the temperature gets too high – usually above 160°F or 180°F depending on the model – the Limit Switch cuts the power to the gas valve to prevent the furnace from cracking or causing a fire.

The Root Cause: Restricted Airflow

A furnace breathes. It pulls cold air in, heats it up, and pushes hot air out. If you choke that airflow, the heat gets trapped inside the unit, and the temperature spikes rapidly.

90% of the time, Code 33 is caused by one of these three things:

1. A Dirty Air Filter: This is the #1 culprit. A clogged filter acts like a brick wall. The blower motor cannot pull enough air through it to cool the heat exchanger.
2. Blocked Return Vents: Did you move a couch or a rug over the big vent in the floor or wall? If the furnace cannot inhale, it overheats.
3. Closed Supply Registers: Homeowners often close vents in unused rooms to “save money.” This actually increases static pressure and causes the furnace to overheat. Keep at least 80% of your vents open.

The Fix

1. Check the Filter: Pull your filter out. If it is grey, matted with dog hair, or caked in dust, replace it immediately.
2. Check Airflow: Ensure all return vents are unblocked.
3. Reset: Once the filter is changed and the unit cools down, the Limit Switch should auto-reset. If the blower motor runs continuously but there is no heat, see the section below on Code 13.

Code 13: Limit Circuit Lockout (The Consequence)

Code 13 (1 Short, 3 Long) is not a separate failure. It is the angry big brother of Code 33.

Technical Definition: Limit Circuit Lockout.

Translation: Your furnace overheated too many times in a row, so the computer shut it down completely.

If the Limit Switch trips (Code 33) three times within a single heating cycle, the Carrier logic board assumes something is critically wrong. To protect the heat exchanger from cracking due to thermal stress, it enters a Hard Lockout.

Hard Lockout vs. Soft Lockout

• Code 33 is a Soft Lockout: The furnace shuts off the burners, runs the fan to cool down, and then tries to fire again automatically.
• Code 13 is a Hard Lockout: The furnace shuts down and will not try to fire again for 3 hours. It forces the blower fan to run continuously to dissipate heat.

The Fix

You cannot simply wait for Code 13 to go away if you want heat now.

1. Solve the Airflow: Perform the steps mentioned in the Code 33 section (change filter, open vents). If you do not fix the airflow, the furnace will just lock out again immediately.
2. Cycle Power: To clear a Hard Lockout, you must interrupt power to the furnace. (See “How to Reset” section below).

Critical Warning: Do not repeatedly reset the power if the furnace keeps hitting Code 33/13. Constant overheating expands and contracts the metal heat exchanger. Eventually, the metal will crack. A cracked heat exchanger can leak fatal carbon monoxide into your home. If you have changed the filter and still get Code 13, you need professional Furnace Repair Services to check for a failed blower motor or a blocked exhaust vent.

Code 34: Ignition Proving Failure (Flame Sensor)

Code 34 (3 Short, 4 Long) is an ignition issue.

Technical Definition: Ignition Proving Failure.

Translation: The furnace turned on the gas and created a fire, but the computer didn’t “see” the fire, so it shut the gas valve for safety.

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The Root Cause: Dirty Flame Sensor

Carrier furnaces use a component called a Flame Sensor. It is a simple metal rod that sits directly in the path of the blue flame.

When the fire hits the rod, it sends a tiny electrical current (microamps) back to the control board, confirming “Fire is present.”

Over time, the metal rod gets coated in carbon, oxidation, and dust. This coating acts as an insulator. The fire hits the rod, but the electrical signal cannot get through the grime. The computer thinks the gas is flowing but there is no fire (a dangerous situation), so it cuts the gas immediately.

The Fix: Clean the Sensor

You can fix this in 5 minutes with a screwdriver and a scouring pad.

1. Turn Off Power: Flip the furnace switch to OFF.
2. Locate the Sensor: It is a single white wire leading to a metal rod on the opposite side of the burner assembly from the igniter (the glowing stick).
3. Remove: Use a 1/4″ nut driver to remove the single screw holding the sensor in place. Pull it out gently.
4. Clean: Use a piece of steel wool, a Scotch-Brite pad, or even a crisp dollar bill. Gently scrub the metal rod until it is shiny and silver. Do not use sandpaper (it leaves grit that melts into glass) and do not touch the rod with your bare fingers (oils cause hot spots).
5. Reinstall: Screw it back in, reattach the wire, and turn the power on.

Carrier Diagnostic Code Cheat Sheet

While 13, 33, and 34 are the most common, here are other codes you might encounter looking through the peep hole.

Code	Flash Pattern	Meaning	Quick Fix
12	1 Short, 2 Long	Blower On After Power Up	This is normal if you just turned the power on. Wait 90 seconds.
14	1 Short, 4 Long	Ignition Lockout	The furnace failed to light 4 times in a row (Code 34 repeated). Reset power and clean the flame sensor.
23	2 Short, 3 Long	Pressure Switch Did Not Open	Check for a blocked drain hose or water in the inducer housing.
31	3 Short, 1 Long	Pressure Switch Did Not Close	The exhaust cannot leave. Check for blocked flue pipe, dead inducer motor, or blocked drain.
Rapid Flash	Constant Fast	Call for Heat	This is Normal Operation. The thermostat is asking for heat.
Solid On	Constant Solid	Control Board Failure	The brain is dead. Requires board replacement.

How to Reset a Carrier Furnace

If you have fixed the root cause (changed the filter or cleaned the sensor), you need to clear the error code from the memory to get the heat back on immediately.

1. Turn Thermostat Off: Go upstairs and set your thermostat to “Off” or turn the temperature way down.
2. Locate the Service Switch: Look for a standard light switch mounted on the side of the furnace or on a ceiling joist nearby. This is the emergency shutoff.
3. Cycle Power: Flip the switch to the OFF position.
4. Wait: Leave it off for at least 30 seconds. This allows the capacitors on the control board to drain.
5. Turn On: Flip the switch back to ON.
6. Watch: Look through the peep hole. The light should flash a “Code 12” (Blower On) and then settle into a rapid heartbeat or solid light as it starts the ignition sequence.

For detailed diagrams and model-specific wiring, you can refer to the official Carrier Residential Manuals database.

Warning: We cannot stress this enough – if you reset the furnace and it immediately overheats and throws Code 33 again, STOP. Do not force it to run. You are risking a cracked heat exchanger or a fire.

FAQ

What does a rapid, continuous flashing light mean?

A red light that pulses quickly and steadily (like a heartbeat) without pausing is usually not an error code. On most Carrier models, a rapid flash indicates a normal “Call for Heat.” It means the thermostat is signaling the furnace to run, and the system is operating correctly. An error code will always have a distinct pause or a combination of short and long flashes.

Why is there no light coming from the furnace peep hole?

If there is no LED light visible at all, the control board is not receiving power. This can be caused by three things:

1. The circuit breaker has tripped.
2. The “Service Switch” (light switch on the side of the unit) is turned off.
3. The blower compartment door is not fully closed. There is a safety “door switch” that cuts all power to the unit if the panel is loose or removed. Push the door firmly to engage the switch.

How long does a Carrier furnace lockout last?

A “Soft Lockout” (like Code 33) typically resets automatically once the internal temperature drops to a safe level. A “Hard Lockout” (Code 13 or 14), caused by repeated failures, lasts for 3 hours. During this time, the furnace will run the blower fan to cool down but will not attempt to ignite. This lockout can be bypassed by turning the power to the furnace off for 30 seconds and then back on.

Can closed vents cause a Code 33 error?

Yes. Closing supply registers in unused rooms increases static pressure in the ductwork. This prevents the blower motor from moving enough air across the heat exchanger, causing the internal temperature to spike and trip the High Limit Switch (Code 33). For the system to function correctly, at least 80% of the home’s vents should remain open.

Where is the error code chart located on the furnace?

Most Carrier furnaces have a diagnostic code sticker located on the inside of the blower compartment door. To view it, turn off the power to the furnace, remove the top and bottom metal panels, and look at the back side of the panels. This chart lists the specific codes for that exact model.

Conclusion

A blinking red light on your Carrier furnace is not a catastrophe; it is a conversation.

• Code 33? Your furnace is suffocating. Change the filter and open the vents.
• Code 13? You ignored Code 33 too long. Fix the airflow and reset the power.
• Code 34? Your furnace has forgotten what fire looks like. Clean the flame sensor.

Most of these issues can be solved in under 15 minutes without any tools other than a screwdriver. However, if you have verified the airflow and cleaned the sensor, but the red light persists, you likely have a failing component like a bad gas valve, a dead inducer motor, or a cracked heat exchanger.

If you are unsure, or if you smell gas, turn the unit off and call for professional help. Contact us to get a certified technician to your home today.