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Carrier gas furnace code 42: Inducer motor fault
Gas furnace, Canada and United States
The inducer motor's speed signal was not detected within 20 seconds of a call for heat, its speed was outside the valid range, or the signal was lost for 5 seconds while running. On 59SU5A and 59CU5A it also shows when the inducer goes over its model's speed limit.
On the status light, this code is 4 short flashes, then 2 long flashes.
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.
Only some models show this code. Check the list below.
What causes it
Ultra-low NOx furnaces
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A wrongly sized vent or a restricted combustion air supply.
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.
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On 59SU5A and 59CU5A, a frozen or blocked drain.
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A failed inducer motor.
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Wrong or loose inducer motor wiring, or a bad connection.
Standard amber-LED furnaces
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A wrongly sized vent or a restricted combustion air supply.
Same check as above: Check the vent and air intake pipes outside are clear
-
A failed inducer motor.
-
Wrong or faulty inducer motor 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
28 models in 6 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 |
| 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 |
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
- 59MN7A modulating condensing gas furnace, installation, start-up, operating and service instructions (59MN7A-12SI) , page 72, 91
- 59MN7B modulating condensing gas furnace, installation, start-up, operating and service instructions (59MN7B-02SI) , page 73
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 |
| 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 |
| 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 |
| 42.1 | No inducer speed at start-up | Call a pro |
| 42.2 | Inducer speed lost while running | Call a pro |
| 42.3 | Inducer over its speed limit | Call a pro |
See all flue or venting fault codes on gas furnaces across brands.