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I have a carrier furnace that has a low speed fan blower motor that will not shut off no matter how long I leave it running. I checked that removal of the G wire at the control board does not change anything. This should rule out the thermostat being an issue. I checked the furnace limit switch and it seems fine with good continuity at the two switch parts (the button part and the actual part that sticks into the hot zone). The heater works fine and will shut itself off when it reaches temp but this only changes the high speed fan. I checked continuity over the fusible link off of the limit switch and it has good continuity. Anyway since the heater works I read that it’s not a problem with the furnace high limit switch. I pulled the low speed fan wire off the control board and that successfully shuts off the low speed fan but then I hear a small humming sound from what might be the HFR or heating fan relay. So is the remaining issue pretty much just the control board or am I missing something I can check? Control board with lo fan disconnected

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  • Did this just start doing this? It could be the relay humming, or the start/run capacitor.
    – Jeff Cates
    Mar 3, 2018 at 22:17
  • @JeffCates the fan hasn't turned itself off in a couple of weeks, so I just pull the plug on the whole thing most of the time. The humming only comes on from the control board when I pull the low speed fan wire out (the red wire in the middle of the picture).
    – James C
    Mar 3, 2018 at 23:41
  • Sounds like maybe the circulation fan relay is bad, or there's a short causing the relay to actuate. You could check it (if you don't mind poking around in the system). Or you could just throw parts at it, and replace the control board.
    – Tester101
    Mar 4, 2018 at 1:30
  • FYI shout out to deydistributing.com who gave me an answer as to what part can replace my old fan control board: ICM271. The manual says that it can replace Carrier/BDP parts 302075-3 CES0110017 CES0110018 HH84AA010 HH84AA011 HH84AA012 HH84AA013 HH84AA020 P771-7002.
    – James C
    Mar 5, 2018 at 16:52

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Dangerous! Don't pull LO FAN plug, it dissipate the heat.

Sounds like LO FAN relay stuck, better replace the whole (blower control) board.

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    Dec 26, 2019 at 1:15

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