Problem: Furnace blower runs fine when heating is on (no AC system installed), but does not activate independently on demand when switched ON from thermostat (i.e., running fan by itself is currently not possible due to a wiring issue).

Furnace model: Carrier 58STA110-10112

Thermostat model: White Rodgers Thermostat 156-5259


  • G terminal in thermostat was not hooked up to the green wire. Connected, expecting the problem to resolve. Issue remains.
  • G terminal on furnace blower is a black wire. There is some kind of miswiring problem here, although I'm not sure what. That's what I'm hoping to get help with: how to re-wire to fix this problem.

Tldr: Furnace blower only works in AUTO with heating and not ON.

Wiring schematics:

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Wiring photos:

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  • Did it ever work?
    – KMJ
    Commented Feb 22 at 6:25
  • It looks like you have a white cable connected to R and W, and a red cable connected to R and G. Where do those two cables go? I'm going to guess they don't both go to the thermostat, in which case the thermostat has only two wires and therefore it cannot control the fan independently. Are you able to replace the thermostat cable? If you have an unfinished basement ceiling and the stat is on a main floor wall, this is relatively easy.
    – jay613
    Commented Feb 22 at 7:04
  • Please add the t-stat make/model
    – FreeMan
    Commented Feb 22 at 13:23
  • There may be a hardwired flame safety/interlock.
    – red_menace
    Commented Feb 22 at 16:53
  • 1
    Edit that info into your question to make sure everyone sees it. It can easily get lost or deleted down here in the comments.
    – FreeMan
    Commented Feb 22 at 17:43

1 Answer 1


Temporarily connect G to R, if the fan starts the problem is not on the controller board.

In that case find the other end of the red cable and connect the red and black wires, if the fan starts now the problem is with whatever is connected to the red cable. If the thermostat is not connected here, well there's your problem!

  • Thanks for getting back. Not super experienced with electrical wiring so just want to double check what to do. To confirm, do I do the following steps? 1. Disconnect both reds from R? 2. Disconnect the black wire from G and connect this to R? And that is it?
    – Unknown
    Commented Feb 23 at 3:58
  • There's no need to disconnect the wires in the red cable, however you may do that if it makes testing easier. I would just use an alligator clip lead to make the temporary connections, but that's the sort of tool I have laying around here. You do you.
    – Jasen
    Commented Feb 23 at 11:24
  • Given the complete mismatch of the cables at the furnace and the thermostat, and given that it works on auto, I'd be very surprised to find that jumping RG fails. I don't know, I'd probably do this anyway because it's so easy, but the additional photo of the thermostat is a smoking gun in the hand of the wiring.
    – jay613
    Commented Feb 24 at 9:18

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