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Carrier gas furnace code 42.3: Inducer over its speed limit
Gas furnace, Canada and United States
The inducer is running past its model-specific speed limit.
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
-
Improper or loose inducer wiring or a bad connection, or a failed inducer motor.
-
A wrongly sized vent, a restricted combustion air supply, or a frozen or blocked drain.
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
22 models in 5 ranges
| Range | Models |
|---|---|
| 58CU0B | 58CU0B040C17-12, 58CU0B060C17-16, 58CU0B080C21-20, 58CU0B100C21-20 |
| 58SU0B | 58SU0B040M17-12, 58SU0B060M17-16, 58SU0B080M21-20, 58SU0B100M21-20 |
| 59CU5B | 59CU5B060C17-16, 59CU5B080C21-20, 59CU5B100C21-22 |
| 59SU5B | 59SU5B040M17-12, 59SU5B060M17-16, 59SU5B080M21-20, 59SU5B100M21-20, 59SU5B100M21-22 |
| 59MN7C | 59MN7C060C17-14, 59MN7C060C21-20, 59MN7C080C17-14, 59MN7C080C21-20, 59MN7C100C21-22, 59MN7C120C24-22 |
Where this comes from
- 58SU0B gas furnace, installation, start-up, operating and service instructions (58SU0B-03SI) , page 24, 39
- 59SU5B gas furnace, installation, start-up, operating and service instructions (59SU5B-03SI) , page 48, 63
- 58CU0B gas furnace, installation, start-up, operating and service instructions (58CU0B-03SI) , page 28, 43
- 59CU5B gas furnace, installation, start-up, operating and service instructions (59CU5B-05SI) , page 49, 66
- 59MN7C gas furnace, installation, start-up, operating and service instructions (59MN7C-04SI) , page 53, 74
- 58SU0B gas furnace, installation, start-up, operating and service instructions (58SU0B-03SI) , page 39
Checked against the manufacturer's manual on 11 October 2026.
Other Carrier gas furnace codes
| Code | Meaning | What you can do |
|---|---|---|
| 42 | Inducer motor fault | Safe checks first, then 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 |
| 42.1 | No inducer speed at start-up | Call a pro |
| 42.2 | Inducer speed lost while running | Call a pro |
| 43.1 | High switch closed before the low switch | Call a pro |
See all flue or venting fault codes on gas furnaces across brands.