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My furnace is not working. I see that the inducer motor (Fasco 70623925) is not spinning (it turns freely when I spin it by hand though). The motor threatens to run when the heat signal is received, but stops within a few milliseconds. I noticed two things:

  1. When I disconnected the inducer motor from the main board and checked the voltage received in the cable from the board (white and black wires) after the heat signal, sometimes it gets 120V (as it should), but sometimes it gets something around 3V only. Why is this happening? Is my board bad?
  2. When I plug the inducer motor to the board, the voltage reading goes up after the heat signal, but then very fast it goes to 0V where it stays. What can cause it?

motoThank you.

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  • does it look like this i.ebayimg.com/images/g/IrwAAOSw9fZeog~X/s-l640.jpg
    – Traveler
    Commented Feb 3, 2023 at 7:59
  • Yes, that's the one I have. Commented Feb 3, 2023 at 13:15
  • How have you determined that the motor "threatens to run" but stops within "a few milliseconds"? A few milliseconds is a mighty short time for a human to register
    – FreeMan
    Commented Feb 3, 2023 at 14:58
  • Hi Freeman.. I determined this visually. I see the inducer trying to turn, but it's only a slight movement, it doesn't turn even 1/50th of a full turn. I believe this happens when voltage is received, but as I mentioned, it goes to 0V very quickly, so the motor doesn't keep turning. Commented Feb 3, 2023 at 15:18
  • I would suspect defective capacitor
    – Traveler
    Commented Feb 3, 2023 at 18:10

4 Answers 4

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The problem was my control board. It had at least one burnt-out relay. After changing the board, everything started working.

Thank you.

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Inducer motors (these days) have the capability of running at various speeds. It's not uncommon that you would read 120v AND sometimes less.

The control board decides the overall voltage being sent to inducer.

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  • Thanks Andrew. When I said "after the heat signal, sometimes it gets 120V (as it should), but sometimes it gets something around 3V only", I mean that when 3V is sent, it never goes up to 120V, staying in 3V voltage all the time (at least until I plug the inducer back, when it goes to 0V). Should an inducer be able to work with this low voltage considering that 120V is the normal? Also, any idea why it goes from 120V or 3V (whichever is been sent) to 0V when I connect the inducer motor to the board? Commented Feb 3, 2023 at 14:23
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Firs you are too focused on the 3 Volt. It is just a phantom voltage.

You have a motor with 3 speeds, that can be only selected by physically changing the wires (not the case).

So far we know what is working in that the thermostat asks for start, the control board acts upon that by sending a signal to the motor, but it also checks for interlocks to continue.

If for example the over temp sensor is reporting signal it will stop the motor and the furnace. Find the over temp sensor and check it.

I could not help further since do not know your actual furnace model, or have schematics to look at it, but give you just general answer. For example, If you have 24V relay on the control board that fires up the motor, does it stays on ?

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Check the pressure switch (may be two of them) - they're usually screwed to to, or nearby, the inducer motor.

I'd still suspect the capacitor as an initial cause; they're cheap to replace.

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