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Carrier gas furnace code 31.1: 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 | Main pressure switch open |
| 59MN7C | Medium pressure switch open when it should be closed |
Main pressure switch open
On 58SB0B/58SB1B, 58SC0B/58SC1B, 58SP0B/58SP1B.
The main pressure switch is open when the control needs it closed. If it stays open, the inducer shuts off for 15 minutes before retrying.
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
-
A restricted or wrongly sized vent, or strong wind at the vent.
-
Disconnected or blocked pressure tubing, or a faulty pressure switch.
-
Low inducer voltage or a failed inducer motor.
-
Not enough combustion air.
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
66 models in 3 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 |
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, 44
Checked against the manufacturer's manual on 11 October 2026.
Medium pressure switch open when it should be closed
On 59MN7C.
The medium heat (MPS) pressure switch input is off when it should be on, so the switch is open when the control needs it closed.
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.
-
Disconnected or blocked pressure tubing, or a faulty pressure switch.
-
A failed inducer motor.
-
A plugged condensate drain or water lying in a sagging vent pipe.
-
Not enough combustion air.
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, 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
6 models in 1 range
| Range | Models |
|---|---|
| 59MN7C | 59MN7C060C17-14, 59MN7C060C21-20, 59MN7C080C17-14, 59MN7C080C21-20, 59MN7C100C21-22, 59MN7C120C24-22 |
Where this comes from
- 59MN7C gas furnace, installation, start-up, operating and service instructions (59MN7C-04SI) , page 53, 74
Checked against the manufacturer's manual on 11 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 |
| 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 |
| 31.2 | 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 |
| 32.1 | Main pressure switch (LPS) open | Safe checks first, then a pro |
| 32.2 | Low pressure switch open | Safe checks first, then a pro |
See all flue or venting fault codes on gas furnaces across brands.