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Carrier gas furnace code 42.2: Inducer speed lost while running

Gas furnace, Canada and United States

The control lost the inducer speed signal after ignition was proved. The SU labels call it no RPM feedback after ignition is proved, while the CU labels say no RPM when the inducer motor is running.

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

  1. Improper or loose inducer wiring or a bad connection, or a failed inducer motor.

    Book a licensed gas technician

  2. A wrongly sized vent, a restricted combustion air supply, or a frozen or blocked drain.

    Book a licensed gas technician

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

  1. Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
  2. Note the code, how often it appears, and the model and serial numbers from the rating label.
  3. Book a licensed gas technician and give them those details.

Models that show this code

22 models in 5 ranges
RangeModels
58CU0B58CU0B040C17-12, 58CU0B060C17-16, 58CU0B080C21-20, 58CU0B100C21-20
58SU0B58SU0B040M17-12, 58SU0B060M17-16, 58SU0B080M21-20, 58SU0B100M21-20
59CU5B59CU5B060C17-16, 59CU5B080C21-20, 59CU5B100C21-22
59SU5B59SU5B040M17-12, 59SU5B060M17-16, 59SU5B080M21-20, 59SU5B100M21-20, 59SU5B100M21-22
59MN7C59MN7C060C17-14, 59MN7C060C21-20, 59MN7C080C17-14, 59MN7C080C21-20, 59MN7C100C21-22, 59MN7C120C24-22

Where this comes from

Checked against the manufacturer's manual on 11 October 2026.

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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
42.3 Inducer over its speed limit 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.