- Home
- Carrier
- gas furnaces
- Code 31
Carrier gas furnace code 31: 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 close, or opened during heating |
| 58CU0A, 58SU0A, 59CU5A, 59SU5A | Pressure transducer did not sense the target pressure |
Pressure switch did not close, or opened during heating
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 that proves the draft inducer is moving enough combustion air did not close, or opened again. Which switch depends on the board. On single-stage boards that do not show code 32 it covers any pressure switch. On boards that also show 32 it is the high or high-heat switch, or its relay. On modulating furnaces it is the medium or high switch, or its relay. Depending on the board, the inducer stops for 15 minutes before a retry, or the control locks out for 3 hours after three failed tries.
On the status light, this code is 3 short flashes, then 1 long flash.
This needs a licensed gas technician. There are a few safe checks you can do first, marked below. Anything inside the unit is a job for a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
What causes it
-
A restricted or wrongly sized vent, or strong wind at the vent.
Check the vent and air intake pipes outside are clearYou can do this
- Find where the furnace vent pipe ends outside the house. A two-pipe (direct-vent) installation also has an air intake pipe ending outside. Check only what you can see from the ground. Do not climb onto the roof or use a ladder. If the vent ends on the roof or goes into a chimney, book a technician to check it.
- Check nothing is blocking or covering the ends of the pipes, in any season, and clear anything stored or growing around them.
- On a condensing furnace with a one-pipe installation, also check the combustion-air pipe next to the furnace is not blocked and nothing is crowding the air openings around the furnace.
- If a pipe is blocked inside, has holes or has come apart, do not run the furnace. Call your dealer. A blocked, holed or disconnected vent pipe can let toxic fumes into your home.
- Find where the furnace vent pipe ends outside the house. A two-pipe (direct-vent) installation also has an air intake pipe ending outside.
-
On condensing furnaces, a plugged condensate drain or water lying in a sagging vent pipe.
-
Disconnected or blocked pressure tubing, or a faulty or mis-wired pressure switch.
-
A faulty inducer motor, or low voltage to it.
-
Not enough combustion air, or an air leak between the burner compartment and the blower compartment.
-
On two-stage and modulating boards, a faulty pressure switch relay on the control, or a mis-wired gas valve.
-
Low inlet gas pressure, on furnaces fitted with a low gas pressure switch.
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
- 58STA/STX single-stage induced-combustion gas furnace, installation, start-up, operating and service instructions (58ST-24SI) , page 43
- 59SC5B single-stage 35-in. condensing gas furnace, installation, start-up, operating and service instructions (59SC5B-05SI) , page 69
- 59SP6A single-stage variable-speed condensing gas furnace, installation, start-up, operating and service instructions (59SP6A-01SI) , page 59
- 59TP6A two-stage variable-speed condensing gas furnace, installation, start-up, operating and service instructions (59TP6A-10SI) , page 73, 90
- 59MN7A modulating condensing gas furnace, installation, start-up, operating and service instructions (59MN7A-12SI) , page 72, 91
Checked against the manufacturer's manual on 13 October 2026.
Pressure transducer did not sense the target pressure
On 58CU0A, 58SU0A, 59CU5A, 59SU5A.
The pressure transducer did not sense its target pressure, or the pressure was out of range. The checks cover the vent, the inducer and the transducer itself.
On the status light, this code is 3 short flashes, then 1 long flash.
This needs a licensed gas technician. There are a few safe checks you can do first, marked below. Anything inside the unit is a job for a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
What causes it
-
A restricted or wrongly sized vent, or strong wind at the vent.
Check the vent and air intake pipes outside are clearYou can do this
- Find where the furnace vent pipe ends outside the house. A two-pipe (direct-vent) installation also has an air intake pipe ending outside. Check only what you can see from the ground. Do not climb onto the roof or use a ladder. If the vent ends on the roof or goes into a chimney, book a technician to check it.
- Check nothing is blocking or covering the ends of the pipes, in any season, and clear anything stored or growing around them.
- On a condensing furnace with a one-pipe installation, also check the combustion-air pipe next to the furnace is not blocked and nothing is crowding the air openings around the furnace.
- If a pipe is blocked inside, has holes or has come apart, do not run the furnace. Call your dealer. A blocked, holed or disconnected vent pipe can let toxic fumes into your home.
- Find where the furnace vent pipe ends outside the house. A two-pipe (direct-vent) installation also has an air intake pipe ending outside.
-
On 59SU5A and 59CU5A, a frozen or blocked drain.
-
Disconnected or blocked pressure tubing, or a faulty pressure transducer.
-
A faulty inducer motor, or low voltage to it.
-
Not enough combustion air.
-
Low inlet gas pressure, on furnaces fitted with a low gas pressure switch.
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 |
|---|---|---|
| 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 |
| 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 |
| 31.3 | Secondary pressure switch lockout after 3 failed trials | Safe checks first, then a pro |
| 31.4 | Loss of pressure before the blower started | Safe checks first, then a pro |
| 31.5 | Loss of pressure after the blower started | Safe checks first, then a pro |
| 31.6 | High pressure switch open | Safe checks first, then a pro |
| 31.7 | High heat only, high switch failed to close in time | Safe checks first, then a pro |
See all flue or venting fault codes on gas furnaces across brands.