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Goodman gas furnace code b9: Low indoor airflow

Gas furnace, Canada and United States

Airflow is lower than the system is asking for. The furnace works at reduced performance or runs on low stage when high stage is expected.

You can often clear this yourself. Work through the causes below in order, most likely first.

What causes it

  1. Blocked filters.

    Check and replace the furnace air filterYou can do this

    1. Turn off the electrical power to the furnace before you take the filter out.
      Never run the furnace without a filter in place.
    2. Find the filter. It sits outside the furnace cabinet, in a return air grille, a filter rack on the side of the furnace, or the duct next to it. If you cannot find or reach it, ask your installer.
    3. Replace a dirty disposable filter with one of the same size that meets UL900 or CAN/ULC-S111. Wash and dry a permanent filter the way its maker describes.
    4. Fit the filter so the air flows through it in the same direction as before, then turn the power back on.
    5. Check the filter at least once a month and clean or replace it when it is dirty. A dirty filter is the most common cause of a furnace not working properly.

    Book a licensed gas technician

  2. Restrictive ductwork or registers that are not fully open.

    Clear blocked supply and return grillesYou can do this

    1. Go round the house and look at every supply register and return air grille.
    2. Move furniture, curtains, clothes, rugs or anything else that covers or blocks them.

    Book a licensed gas technician

  3. Ductwork that is too small for the system.

    Book a licensed gas technician

How to read this code on your unit

The code shows on the two-character display on the furnace control board, which you can see through the sight glass on the blower compartment. On means normal operation. A blank display means the board has no power or has failed. A ComfortNet thermostat, if fitted, shows the same codes with a short message and uses EE for an internal fault.

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

66 models in 4 ranges
RangeModels
GMVC8 / GCVC8 / AMVC8 / ACVC8GMVC80603BN, GMVC80603BX, GMVC80604BN, GMVC80604BX, GMVC80803BN, GMVC80803BX, GMVC80804CN, GMVC80804CX, GMVC80805CN, GMVC80805CX, GMVC80805DN, GMVC80805DX, GMVC81005CN, GMVC81005CX, AMVC80603BN, AMVC80603BX, AMVC80604BN, AMVC80604BX, AMVC80803BN, AMVC80803BX, AMVC80804CN, AMVC80804CX, AMVC80805CN, AMVC80805CX, AMVC80805DN, AMVC80805DX, AMVC81005CN, AMVC81005CX
GMVC96 / GCVC96 / AMVC96 / ACVC96GMVC960403BN, GMVC960603BN, GMVC960803BN, GMVC960804CN, GMVC961005CN, GMVC961005DN, GMVC961205DN, GCVC960403BN, GCVC960603BN, GCVC960804CN, GCVC961005CN, GCVC961205DN, AMVC960403BN, AMVC960603BN, AMVC960803BN, AMVC960804CN, AMVC961005CN, AMVC961005DN, AMVC961205DN, ACVC960403BN, ACVC960603BN, ACVC960804CN, ACVC961005CN, ACVC961205DN
GMVM96 / GCVM96 / AMVM96 / ACVM96GMVM960603BX, GMVM960805CX, GMVM961005DX, GMVM961155DX
GMVM97 / GCVM97 / AMVM97 / ACVM97GMVM970603BN, GMVM970803BN, GMVM970804CN, GMVM971005CN, GMVM971205DN, AMVM970603BN, AMVM970803BN, AMVM970804CN, AMVM971005CN, AMVM971205DN

Where this comes from

Checked against the manufacturer's manual on 13 October 2026.

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Other Goodman gas furnace codes

Code Meaning What you can do
b3 Blower motor limiting its power, temperature or speed You can often fix it
b4 Blower motor tripped (lost rotor control or high current) Safe checks first, then a pro
b5 Blower motor fails to start (locked rotor) Call a pro
b6 Blower motor shut down for voltage or overheating Call a pro
b7 Blower motor lacks the data it needs to run Call a pro
d0 No shared data loaded on the control board Call a pro
d4 Invalid memory card data Call a pro
E0 Lockout after three failed ignition attempts Safe checks first, then a pro
E1 Low-stage pressure switch closed at start of heating Call a pro
E1A Onboard return air sensor shorted Call a pro
Eb3 Blower motor limiting its power, temperature or speed You can often fix it
Eb9 Low indoor airflow You can often fix it