Troubleshooting Error Code 33 On Carrier Furnace

Carrier gas furnaces offer energy-efficient heating with variable speed blowers and humidity control. These furnaces include many safety features, which include an “error code 33” notification that typically relates to restricted airflow and overheating.

Troubleshooting an “error code 33” notification can start with the homeowner. First, reset the control board by turning it off for a few seconds, and then turning it back on. If the error code remains, change the filter next. More labor-intensive troubleshooting involves checking or replacing the cold air baffle or secondary heat exchanger. If the problem persists, professional service is needed.

This article will provide further information about what the “error code 33” notification means, as well as how to troubleshoot the problem on your own. Addressing this error code will help ensure that your equipment is safely and efficiently running.

What Does Error Code 33 Mean?

Furnaces have a control board that regulates the administration of gas to power the flame, operates the fan, detects airflow leaks, and symptoms of overheating. The control board often works in conjunction with blinking lights to indicate the type of problem. 

“Error code 33” will appear on a Carrier furnace when there is an issue with the airflow or the risk of overheating. Often a yellow light comes on with this error.

This is a limit circuit fault safety feature that indicates the switch to the flame rollout is off, turning off the furnace. It protects the heat exchanger from potentially cracking, which would result in an expensive repair. 

A cracked heat exchanger could also pose serious threats to the people in your home. It could allow carbon monoxide, nitrous oxide, and sulfur dioxide into your home causing illness and even death in extreme cases.  

“Error code 33” is considered a high-limit lockout because the furnace is overheating several times in a row. The furnace cannot be turned back on until the problem is addressed.

Typically this code indicates that the furnace needs to be reset, a part needs to be maintained or replaced for proper airflow, or the filter needs to be changed. 

Depending upon the cause of the error code, this will require a DIY task or professional service.

Common Fixes For Error Code 33 

“Error code 33” can save your life, and it is not something to ignore if it appears. 

Thankfully, there are multiple fixes for this problem. Some of them can be simply performed by you, and others can be accomplished with the help of a service professional.

Reset Your Furnace 

The first thing is to try resetting your furnace. This can save you money and prevent the need for an immediate service call.  

What To Do

  1. To figure out the error code, look through the glass window on the front door of the furnace. Take note of the pattern of blinking to determine the code.
    1. Consider taking a video of the error code or blinking light pattern for future reference.
  2. Remove the front panel door, and locate the list of codes that are typically on the backside.
    1. If desired, you can call your service professional and inquire about the code and they can tell you what to do over the phone for free.
  3. Put the door back on if you removed it.
  4. Locate the switch, which looks just like a light switch, and turn it off.
    1. It typically is on the furnace to the left or right of the front.
  5. Leave the switch in the off position for 3 seconds and then turn it back on. This essentially reboots the furnace’s internal control system.
  6. If the furnace turns back on without the error code, then you have successfully reset the furnace.
    1. Adjust the thermostat if necessary to send a “call” for heat to test it.  
    2. Since furnace models can vary, 3 seconds may not be enough time to reset it. So, flip the switch off again, but this time for 20 seconds.

If the furnace will not reset or continues to flash the error code every time it turns on, try the following strategies below or call a service professional.

Replace The Secondary Heat Exchanger

Replacing the secondary heat exchanger can allow you to get a few more years of use out of it instead of replacing the entire furnace. 

If you feel up to this task, you can do this on your own with some basic tools. All you need is a screwdriver, pinchers, a razorblade, high-temperature silicone, and a drill with an extension bit.

What To Do

  1. Remove the front panel, typically held on by two screws.
  2. Shut off all power and gas supplies to the furnace. However, if you are unsure if gas or electricity is still flowing, call a service professional first to ask how to do this.
  3. If possible, locate the user manual for your model of furnace to help you identify parts.
    1. It is highly recommended that you take pictures of what it looks like before you remove parts so that you can refer to them when you reassemble everything.
  4. Take out the combustion blower, burner box, electrical box, condensate trap, and old heat exchanger.
    1. Gently use tools and your fingers to remove all wiring, tubing, and screws that are connected to these parts. 
    2. If the furnace has been working hard or overheating, you will see soot, rust, and signs of wear and tear. 
  5. Once you take out the “housing” for all of these parts, take off the back plate. Remove any old silicone and rubber along with the baffle.
  6. Lift out the old heat exchanger. Inspect for signs of pitted damage or rust to see if anything else needs to be replaced or cleaned up. 
  7. Refer to any other instructions included with your new heat exchanger to put it on correctly. This includes new rubber sealing (included with the new heat exchanger).
  8. Seal it in place with high-temperature silicone.
  9. Reassemble the entire unit, making sure panels, shields, wires, tubing, and so on are lined up and secure.
    1. Typically the frame will have notches and screw holes to help line it up.
  10. Turn the electricity and gas line back on
  11. Use your thermostat to test the system.

This video offers a visual representation of how to replace the secondary heat exchanger in a Carrier furnace model:

Check The Cold Air Baffle

The cold air baffle controls the flow and amount of air throughout the HVAC system. This is made of metal that is slightly smaller than the diameter of the connecting duct.

Baffles are adjustable to control and balance the airflow. If it becomes dirty, clogged, or misaligned, it can cause issues with the airflow, thus causing the “error code 33” to activate.

The baffle can be removed, cleaned, and realigned to allow air to flow well.

Replacing The Baffle

Add a new and efficient cold air baffle if cleaning and alignment do not resolve your issue.

The cold air baffle is connected to the secondary heat exchanger. Follow the steps for replacing a secondary heat exchanger mentioned above to access this part.

Once you have the old baffle off, you add the new one by simply using the rubber sealing strips, and lining up the screws to attach it to the secondary heat exchanger. 

Refer to the instructions to adjust it for airflow as needed.

In the rare case it is missing one, call customer service for the Carrier brand to get one.

Change The Air Filter

If an air filter is clogged, full, or inserted backward, it can reduce airflow, setting off “error code 33”. 

Additionally, if you use a filter that has a higher Minimum Efficiency Reporting Values (MERV)-rating, the filter material is denser. This density can restrict airflow and cause your furnace to work harder. 

Generally, MERV ratings of 6 to 8 are suitable for furnace filters.

What To Do

  1. Turn off the furnace switch.
  2. Make sure all vents and return registers are open.
  3. Remove the barrier or door that holds the filter in place.
  4. Pull out and discard the old filter.
  5. Locate the printed arrow on the side of the new filter.
  6. Insert the new filter so that the arrow is pointing toward the furnace.
  7. Put the door or barrier back in place over the filter.

Check the filter every 6 months for dust, debris, and buildup. If you have pets check it more frequently. 

Typically, a filter is replaced every 8 to 12 months for Carrier models.

When Should You Call A Professional?

You should call a service professional when you have tried various strategies and you are still getting the error 33 code.  

It is recommended that the general homeowner reset the furnace by flipping the switch and changing the filter.

However, many people are not comfortable completely taking apart a furnace to replace the secondary heat exchanger or baffle. In some cases, this could invalidate any warranties that you have. There is a risk of further damage or injury from electrical or gas lines. 

In these cases, a service professional is trained to diagnose and safely remedy furnace issues.

How Much Will It Cost To Fix Error Code 33?

The cost of fixing a Carrier furnace with an “error code 33” yourself is considerably cheaper than using a professional.  

The table below gives general cost comparisons. 

Keep in mind these prices can fluctuate and vary depending on where you live. If it is after hours or on a weekend, the prices of a professional can be even higher.

Type Of FixDIYProfessional
Reset furnacefreeService call: $100 to $250
Replace secondary heat exchangerFree labor; $1500 to $2500 replacement part$65 to $150 hourly labor plus $1500 to $2500 replacement cost
Check the cold air baffleFree labor to clean or adjust (replacement baffle: $30 to $50)Service call or hourly labor: $65 or more (replacement baffle: $30 to $50)
Change the air filterFree labor; filter cost: $50 to $80Service call: $100 to $20, plus cost of filter ($50 to $80)


If “error code 33” (or any other error codes) appears on your Carrier furnace it is a safety feature that turns the furnace off. This is typically due to restricted airflow, which can result in overheating or a cracked heat exchanger.

Before spending money on a service call, your furnace’s control board may need a simple reset. Or, it might be time to replace the filter. 

If these two methods do not work, then the secondary heat exchanger or cold air baffle may need to be replaced or adjusted.

Accessing the heat exchanger and baffle can be a difficult task because it requires removing many parts of the furnace. If this is beyond your DIY abilities, it is best to contact a professional HVAC service.

Leave a Comment