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- Code 23.1
Carrier gas furnace code 23.1: 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 |
|---|---|
| 58SB0B/58SB1B, 58SC0B/58SC1B, 58SP0B/58SP1B, 59SC2E, 59SC6A | Main pressure switch stuck closed |
| 59CU5B | Over-pressure at start-up |
Main pressure switch stuck closed
On 58SB0B/58SB1B, 58SC0B/58SC1B, 58SP0B/58SP1B, 59SC2E, 59SC6A.
The main pressure switch is closed when it should be open, so the control will not start heating. The 58SP0B label calls it the PS stuck closed.
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.
Only some models show this code. Check the list below.
What causes it
-
Blocked pressure tubing holding the switch closed.
-
A pressure switch stuck closed.
How to read this code on your unit
The three-digit display sits on the furnace control behind the blower door. Through the sight glass in the door you see only the status light, which flashes the first two digits of the code. 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 a 33.x code. Write down what you see before you open the door or switch the furnace off. The full code with its third digit shows on the control display only with the door removed and the door switch closed, which is a job for a technician. A technician can also look back at the last 7 faults in the FLt menu. Stored codes are erased by themselves after 72 hours of powered operation after the last fault, and the display shows non when no fault is stored.
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
86 models in 5 ranges
| Range | Models |
|---|---|
| 58SB0B/58SB1B | 58SB0B045M14-12, 58SB0B045M17-14, 58SB0B070M14-12, 58SB0B070M17-12, 58SB0B070M17-16, 58SB0B070M21-16, 58SB0B090M17-14, 58SB0B090M21-16, 58SB0B090M21-20, 58SB0B090M24-20, 58SB0B110M21-20, 58SB0B110M24-20, 58SB0B135M24-20, 58SB0B155M24-20, 58SB1B045M14-12, 58SB1B045M17-14, 58SB1B070M14-12, 58SB1B070M17-12, 58SB1B070M17-16, 58SB1B070M21-16, 58SB1B090M17-14, 58SB1B090M21-16, 58SB1B090M21-20, 58SB1B090M24-20, 58SB1B110M21-20, 58SB1B110M24-20, 58SB1B135M24-20, 58SB1B155M24-20 |
| 58SC0B/58SC1B | 58SC0B045M14-12, 58SC0B045M17-14, 58SC0B070M14-12, 58SC0B070M17-12, 58SC0B070M17-16, 58SC0B070M21-16, 58SC0B090M17-14, 58SC0B090M21-16, 58SC0B090M21-20, 58SC0B110M21-20, 58SC0B110M24-20, 58SC0B135M24-20, 58SC1B045M14-12, 58SC1B045M17-14, 58SC1B070M14-12, 58SC1B070M17-12, 58SC1B070M17-16, 58SC1B070M21-16, 58SC1B090M17-14, 58SC1B090M21-16, 58SC1B090M21-20, 58SC1B110M21-20, 58SC1B110M24-20, 58SC1B135M24-20 |
| 58SP0B/58SP1B | 58SP0B045V14-12, 58SP0B070V14-12, 58SP0B070V17-16, 58SP0B090V17-16, 58SP0B090V21-20, 58SP0B110V21-22, 58SP0B135V24-22, 58SP1B045V14-12, 58SP1B070V14-12, 58SP1B070V17-16, 58SP1B090V17-16, 58SP1B090V21-20, 58SP1B110V21-22, 58SP1B135V24-22 |
| 59SC2E | 59SC2E040M14-12, 59SC2E040M17-12, 59SC2E060M14-12, 59SC2E060M17-14, 59SC2E080M17-16, 59SC2E080M21-20, 59SC2E100M21-20, 59SC2E100M21-22, 59SC2E120M24-20 |
| 59SC6A | 59SC6A026M14-10, 59SC6A040M14-10, 59SC6A040M17-12, 59SC6A060M14-12, 59SC6A060M17-16, 59SC6A080M17-16, 59SC6A080M21-20, 59SC6A100M21-20, 59SC6A100M21-22, 59SC6A120M24-22, 59SC6A140M24-22 |
Where this comes from
- 58SB0B gas furnace, installation, start-up, operating and service instructions (58SB0B-01SI) , page 31, 43
- 58SP0B gas furnace, installation, start-up, operating and service instructions (58SP0B-01SI) , page 30
- 59SC2E gas furnace, installation, start-up, operating and service instructions (59SC2E-03SI) , page 53, 69
Checked against the manufacturer's manual on 11 October 2026.
Over-pressure at start-up
On 59CU5B.
The pressure transducer senses more than 0.15 inches of water column at start-up (over 0.65 volts DC), indicating over-pressure at start-up.
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.
Only some models show this code. Check the list below.
What causes it
-
Blocked pressure tubing.
How to read this code on your unit
The three-digit display sits on the furnace control behind the blower door. Through the sight glass in the door you see only the status light, which flashes the first two digits of the code. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 4 long flashes is a 34.x code. Write down what you see before you open the door or switch the furnace off. The full code with its third digit shows on the control display only with the door removed and the door switch closed, which is a job for a technician. A technician can also look back at the last 7 faults in the FLt menu. Stored codes are erased by themselves after 72 hours, and the display shows non when no fault is stored.
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
3 models in 1 range
| Range | Models |
|---|---|
| 59CU5B | 59CU5B060C17-16, 59CU5B080C21-20, 59CU5B100C21-22 |
Where this comes from
- 59CU5B gas furnace, installation, start-up, operating and service instructions (59CU5B-05SI) , page 49, 67
Checked against the manufacturer's manual on 27 October 2026.
Other Carrier gas furnace codes
| Code | Meaning | What you can do |
|---|---|---|
| 23 | Flue or venting fault (depends on model) | Call 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 |
| 23.2 | 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 |
| 31.2 | Flue or venting fault (depends on model) | Depends on your model |
See all flue or venting fault codes on gas furnaces across brands.