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Carrier gas furnace code 23: Flue or venting fault
Gas furnace, Canada and United States
This code means one of two things, depending on your model. Find your model family below, then read the section for it. The model number is on the rating label on the side of the unit.
Find your model family
| If you have | It means |
|---|---|
| 58CTA/CTX, 58CVA/CVX, 58DLA/DLX, 58SB0A/58SB1A, 58SC0A/58SC1A, 58SP0A/58SP1A, 58STA/STX, 58TN0A/58TN1A, 58TP0A/58TP1A, 59MN7A, 59MN7B, 59SC2A, 59SC2B and 59SC2C, 59SC2D, 59SC5A, 59SC5B, 59SP5A, 59SP6A, 59TN6A, 59TN6B, 59TP5A, 59TP6A, 59TP6B | Pressure switch did not open |
| 58CU0A, 58SU0A, 59CU5A, 59SU5A | Pressure transducer reads too high at start-up |
Pressure switch did not open
On 58CTA/CTX, 58CVA/CVX, 58DLA/DLX, 58SB0A/58SB1A, 58SC0A/58SC1A, 58SP0A/58SP1A, 58STA/STX, 58TN0A/58TN1A, 58TP0A/58TP1A, 59MN7A, 59MN7B, 59SC2A, 59SC2B and 59SC2C, 59SC2D, 59SC5A, 59SC5B, 59SP5A, 59SP6A, 59TN6A, 59TN6B, 59TP5A, 59TP6A, 59TP6B.
A pressure switch stayed closed when the control expected it to be open. The checks are for a switch stuck closed or a blocked pressure tube.
On the status light, this code is 2 short flashes, then 3 long flashes.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
What causes it
-
A pressure switch is stuck closed.
-
The pressure switch tube is blocked.
How to read this code on your unit
The amber status light (red on some older boards) on the furnace control shows through a small window in the blower access door. A steady light means the control has power and no fault to show. A code is two digits. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 3 long flashes is code 33, and 1 short then 1 long is code 11. Write the code down before you open the door or switch the furnace off.
Booking a licensed gas technician
Book a licensed gas technicianlicensed gas technician
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gas technician and give them those details.
Models that show this code
312 models in 22 ranges
| Range | Models |
|---|---|
| 58CTA/CTX | 58CTA045-08, 58CTA045-12, 58CTA070-08, 58CTA070-12, 58CTA070-16, 58CTA090-14, 58CTA090-16, 58CTA090-20, 58CTA110-12, 58CTA110-16, 58CTA110-22, 58CTA135-16, 58CTA135-22, 58CTA155-20, 58CTX045-08, 58CTX045-12, 58CTX070-08, 58CTX070-12, 58CTX070-16, 58CTX090-14, 58CTX090-16, 58CTX090-20, 58CTX110-12, 58CTX110-16, 58CTX110-22, 58CTX135-16, 58CTX135-22, 58CTX155-20 |
| 58CVA/CVX | 58CVA070-12, 58CVA090-16, 58CVA110-20, 58CVA135-22, 58CVA155-22, 58CVX070-12, 58CVX090-16, 58CVX110-20, 58CVX135-22, 58CVX155-22 |
| 58DLA/DLX | 58DLA045-08, 58DLA045-12, 58DLA070-08, 58DLA070-12, 58DLA070-16, 58DLA090-14, 58DLA090-16, 58DLA090-20, 58DLA110-12, 58DLA110-16, 58DLA110-22, 58DLA135-16, 58DLA135-22, 58DLA155-20, 58DLX045-08, 58DLX045-12, 58DLX070-08, 58DLX070-12, 58DLX070-16, 58DLX090-14, 58DLX090-16, 58DLX090-20, 58DLX110-12, 58DLX110-16, 58DLX110-22, 58DLX135-16, 58DLX135-22, 58DLX155-20 |
| 58SB0A/58SB1A | 58SB0A045E14-12, 58SB0A045E17-12, 58SB0A070E14-12, 58SB0A070E17-12, 58SB0A070E17-16, 58SB0A070E21-16, 58SB0A090E17-14, 58SB0A090E21-16, 58SB0A090E21-20, 58SB0A090E24-20, 58SB0A110E21-20, 58SB0A110E24-20, 58SB0A135E24-20, 58SB0A155E24-20, 58SB1A045E14-12, 58SB1A045E17-12, 58SB1A070E14-12, 58SB1A070E17-12, 58SB1A070E17-16, 58SB1A070E21-16, 58SB1A090E17-14, 58SB1A090E21-16, 58SB1A090E21-20, 58SB1A090E24-20, 58SB1A110E21-20, 58SB1A110E24-20, 58SB1A135E24-20, 58SB1A155E24-20 |
| 58SC0A/58SC1A | 58SC0A045E14-12, 58SC0A045E17-12, 58SC0A070E14-12, 58SC0A070E17-12, 58SC0A070E17-16, 58SC0A070E21-16, 58SC0A090E17-14, 58SC0A090E21-16, 58SC0A090E21-20, 58SC0A090E24-20, 58SC0A110E21-20, 58SC0A110E24-20, 58SC0A135E24-20, 58SC1A045E14-12, 58SC1A045E17-12, 58SC1A070E14-12, 58SC1A070E17-12, 58SC1A070E17-16, 58SC1A070E21-16, 58SC1A090E17-14, 58SC1A090E21-16, 58SC1A090E21-20, 58SC1A090E24-20, 58SC1A110E21-20, 58SC1A110E24-20, 58SC1A135E24-20 |
| 58SP0A/58SP1A | 58SP0A045V14-12, 58SP0A045V17-16, 58SP0A070V17-16, 58SP0A070V21-20, 58SP0A090V21-20, 58SP0A090V24-20, 58SP0A110V24-22, 58SP1A045V14-12, 58SP1A045V17-16, 58SP1A070V17-16, 58SP1A070V21-20, 58SP1A090V21-20, 58SP1A090V24-20, 58SP1A110V24-22 |
| 58STA/STX | 58STA045-08, 58STA045-12, 58STA070-08, 58STA070-12, 58STA070-16, 58STA090-14, 58STA090-16, 58STA090-20, 58STA110-12, 58STA110-16, 58STA110-22, 58STA135-16, 58STA135-22, 58STA155-20, 58STX045-08, 58STX045-12, 58STX070-08, 58STX070-12, 58STX070-16, 58STX090-14, 58STX090-16, 58STX090-20, 58STX110-12, 58STX110-16, 58STX110-22, 58STX135-16, 58STX135-22, 58STX155-20 |
| 58TN0A/58TN1A | 58TN0A045C17-12, 58TN0A070C14-12, 58TN0A070C17-16, 58TN0A070C21-20, 58TN0A090C17-16, 58TN0A090C21-20, 58TN0A110C21-20, 58TN0A135C24-22, 58TN1A045C17-12, 58TN1A070C14-12, 58TN1A070C17-16, 58TN1A070C21-20, 58TN1A090C17-16, 58TN1A090C21-20, 58TN1A110C21-20, 58TN1A135C24-22 |
| 58TP0A/58TP1A | 58TP0A045V14-12, 58TP0A070V14-12, 58TP0A070V17-16, 58TP0A090V17-16, 58TP0A090V21-20, 58TP0A110V21-22, 58TP0A135V24-22, 58TP1A045V14-12, 58TP1A070V14-12, 58TP1A070V17-16, 58TP1A090V17-16, 58TP1A090V21-20, 58TP1A110V21-22, 58TP1A135V24-22 |
| 59MN7A | 59MN7A060V17-14, 59MN7A080V17-14, 59MN7A060V21-20, 59MN7A080V21-20, 59MN7A100V21-22, 59MN7A120V24-22 |
| 59MN7B | 59MN7B060C17-14, 59MN7B060C21-20, 59MN7B080C17-14, 59MN7B080C21-20, 59MN7B100C21-22, 59MN7B120C24-22 |
| 59SC2A, 59SC2B and 59SC2C | 59SC2A040E14-10, 59SC2A060E14-10, 59SC2A060E17-14, 59SC2A080E17-16, 59SC2A080E21-20, 59SC2A100E21-14, 59SC2A100E21-20, 59SC2A120E24-20, 59SC2B040E14-10, 59SC2B060E14-12, 59SC2B040E17-12, 59SC2B060E17-16, 59SC2B080E17-16, 59SC2B080E21-20, 59SC2B100E21-16, 59SC2B100E21-22, 59SC2B120E24-22, 59SC2C040E14-10, 59SC2C060E14-12, 59SC2C040E17-12, 59SC2C060E17-16, 59SC2C080E17-16, 59SC2C080E21-20, 59SC2C100E21-16, 59SC2C100E21-20, 59SC2C120E24-20 |
| 59SC2D | 59SC2D040E14-10, 59SC2D040E17-12, 59SC2D060E14-12, 59SC2D060E17-14, 59SC2D080E17-16, 59SC2D080E21-20, 59SC2D100E21-20, 59SC2D100E21-22, 59SC2D120E24-20 |
| 59SC5A | 59SC5A026E14-08, 59SC5A040E14-10, 59SC5A060E14-10, 59SC5A040E17-12, 59SC5A060E17-14, 59SC5A080E17-16, 59SC5A080E21-20, 59SC5A100E21-14, 59SC5A100E21-20, 59SC5A120E24-20, 59SC5A140E24-20 |
| 59SC5B | 59SC5B026E14-10, 59SC5B040E14-10, 59SC5B040E17-12, 59SC5B060E14-12, 59SC5B060E17-16, 59SC5B080E17-16, 59SC5B080E21-20, 59SC5B100E21-20, 59SC5B100E21-22, 59SC5B120E24-22, 59SC5B140E24-22 |
| 59SP5A | 59SP5A026E14-10, 59SP5A040E14-10, 59SP5A060E14-12, 59SP5A040E17-12, 59SP5A060E17-14, 59SP5A080E17-16, 59SP5A080E21-20, 59SP5A100E21-20, 59SP5A120E24-22 |
| 59SP6A | 59SP6A040V14-10, 59SP6A040V17-12, 59SP6A060V14-12, 59SP6A060V17-14, 59SP6A080V17-16, 59SP6A080V21-20, 59SP6A100V21-20, 59SP6A120V24-22 |
| 59TN6A | 59TN6A060V17-14, 59TN6A080V17-14, 59TN6A080V21-20, 59TN6A100V21-22, 59TN6A120V24-22 |
| 59TN6B | 59TN6B060C17-14, 59TN6B080C17-14, 59TN6B080C21-20, 59TN6B100C21-22, 59TN6B120C24-22 |
| 59TP5A | 59TP5A040E14-10, 59TP5A060E14-12, 59TP5A040E17-12, 59TP5A060E17-14, 59TP5A080E17-16, 59TP5A080E21-20, 59TP5A100E21-20, 59TP5A120E24-22 |
| 59TP6A | 59TP6A040V14-10, 59TP6A040V14-12, 59TP6A060V17-12, 59TP6A060V17-14, 59TP6A080V17-16, 59TP6A080V21-20, 59TP6A100V21-20, 59TP6A120V24-22 |
| 59TP6B | 59TP6B040V14-10, 59TP6B040V17-12, 59TP6B060V14-12, 59TP6B060V17-14, 59TP6B080V17-16, 59TP6B080V21-20, 59TP6B100V21-20, 59TP6B120V24-22 |
Where this comes from
- 59SC5A single-stage condensing gas furnace, installation, start-up, operating and service instructions (59SC5A-13SI) , page 67, 83
- 59TP6A two-stage variable-speed condensing gas furnace, installation, start-up, operating and service instructions (59TP6A-10SI) , page 90
Checked against the manufacturer's manual on 11 October 2026.
Pressure transducer reads too high at start-up
On 58CU0A, 58SU0A, 59CU5A, 59SU5A.
At start-up the furnace's pressure transducer read more than 0.15 in. w.c. The checks are for blocked tubing or transducer vent holes, a faulty transducer, or its wiring.
On the status light, this code is 2 short flashes, then 3 long flashes.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
What causes it
-
Something is blocking the pressure tubing or the transducer's vent holes.
-
The pressure transducer is faulty.
-
A fault in the transducer wiring.
How to read this code on your unit
The status light is on the furnace control board. On 58CU0A and 59CU5A furnaces it shows through the sight glass in the blower access door. On 58SU0A and 59SU5A furnaces it is read with the blower door off, which is a job for a technician. A steady light means the control has power and no fault to show. A code is two digits. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 1 long flash is code 31. Write the code down before you open the door or switch the furnace off.
Booking a licensed gas technician
Book a licensed gas technicianlicensed gas technician
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gas technician and give them those details.
Models that show this code
16 models in 4 ranges
| Range | Models |
|---|---|
| 58CU0A | 58CU0A040C17-12, 58CU0A060C17-16, 58CU0A080C21-20, 58CU0A100C21-20 |
| 58SU0A | 58SU0A040E17-12, 58SU0A060E17-16, 58SU0A080E21-20, 58SU0A100E21-20 |
| 59CU5A | 59CU5A060C17-16, 59CU5A080C21-20, 59CU5A100C21-22 |
| 59SU5A | 59SU5A040E17-12, 59SU5A060E17-16, 59SU5A080E21-20, 59SU5A100E21-20, 59SU5A100E21-22 |
Where this comes from
- 58SU0A 80% single-stage ultra-low NOx gas furnace, installation, start-up, operating and service instructions (58SU0A-06SI) , page 33
- 59SU5A single-stage ultra-low NOx condensing gas furnace, installation, start-up, operating and service instructions (59SU5A-05SI) , page 51
- 59CU5A single-stage communicating ultra-low NOx condensing gas furnace, installation, start-up, operating and service instructions (59CU5A-01SI) , page 65
Checked against the manufacturer's manual on 9 October 2026.
Other Carrier gas furnace codes
| Code | Meaning | What you can do |
|---|---|---|
| 31 | Flue or venting fault (depends on model) | Safe checks first, then a pro |
| 32 | Low or low-heat pressure switch did not close, or opened again | Safe checks first, then a pro |
| 42 | Inducer motor fault | Safe checks first, then a pro |
| 43 | Lower pressure switch open while a higher one is closed | Call a pro |
| 14.4 | Lockout after repeated pressure switch openings | Safe checks first, then a pro |
| 23.1 | Flue or venting fault (depends on model) | Call a pro |
| 23.2 | Secondary pressure switch stuck closed | Call a pro |
| 23.3 | Pressure transducer senses over-pressure | Call a pro |
| 23.4 | Low pressure switch stuck closed | Call a pro |
| 23.5 | High pressure switch stuck closed | Call a pro |
| 23.6 | Medium pressure switch stuck closed | Call a pro |
| 31.1 | Flue or venting fault (depends on model) | Depends on your model |
See all flue or venting fault codes on gas furnaces across brands.