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My furnace's inducer motor (Fasco 70623925) has a capacitor that does not specify the tolerance. It only says 4uF, 370vac, 50/60Hz. How can I find out its tolerance? I'm getting 3.7uF with a multimeter when I disconnect it from the inducer and 6.1uF when plugged to the inducer (furnace is off). Is the capacitor good? My furnace is not working. The inducer motor threatens to run when the heat signal is received, but stops within a few milliseconds.

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  • Listing the brand/model of the furnace might enable someone to find documentation which would specify it. You can edit that into your question.
    – FreeMan
    Feb 2, 2023 at 15:10
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    Is the motor still getting voltage after that initial pulse? Does it turn freely when you spin it by hand? Usually motor capacitors don't need to be very precise; they're just there to introduce a bit of delay so the motor starts spinning in the right direction.
    – keshlam
    Feb 2, 2023 at 15:11
  • I added the brand/model of the inducer motor: Fasco 70623925. Feb 2, 2023 at 15:23
  • Yes, it is receiving 120V and the motor turns freely when I spin it by hand. Feb 2, 2023 at 15:25
  • I'm sure for an application like this the tolerance is no better +/- 20 %. Remember, the wider the tolerance, the cheaper the part.
    – SteveSh
    Feb 2, 2023 at 15:28

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Typically a non-explicitly specified capacitor or resistor is +/- 20%

Out of circuit is the only way to reliably measure the device capacitance, and you're well within tolerance on that.

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