I am trying to install a new wifi thermostat in my home and need to locate the common terminal for my air handler. I have a Carrier model FB4CNF030. It's a new construction home and I have the blue common run but it is not connected at the air handler side


I've referred to the post at How can I add a "C" wire to my thermostat? and understand the purpose of the common but still don't know where to hook it up. I can't find the wiring schematic for this unit.

Any suggestions are greatly appreciated.

  • If you share a link to the photo in a comment, a higher reputation user can edit it into your post.
    – Tester101
    Commented May 29, 2013 at 11:09
  • The photo is at alcotechnologies.com/images/airhandler2.jpg
    – techguy817
    Commented May 30, 2013 at 12:16
  • @techguy817 I don't see the control board in this photo.. I suspect it is in the lower compartment, and the yellow/white/brown/blue wires in the bottom right will lead you to it.
    – gregmac
    Commented May 30, 2013 at 18:37
  • The wires in the bottom right seem to lead to a motor down below.
    – techguy817
    Commented Jun 1, 2013 at 1:37

3 Answers 3


There is a set of connections in the air handler that usually includes the "C" terminal, and it should be labelled (though it may be hard to read depending on the location of the control board and/or wiring/etc in the way).

One way to tell for sure is that while the furnace is not running, using a multi-meter you can measure between the "C" and "R" (or Rh or Rc terminals) and you should see 24VAC. R (or Rh and Rc) will already be going to the thermostat.

As an example, on your furnace, from the manual:

enter image description here

The colors may match, but don't count on it. The sure-fire way is to check using a multi-meter as I said above.

  • According to my manual at managemylife.com/mmh/lis_pdf/INSTL/L1003135.pdf on page 6, there is a C terminal at the transformer block where a yellow wire is coming off. I see this in my setup but the manual refers to brown as the common.
    – techguy817
    Commented Jun 1, 2013 at 2:06
  • I hooked up the blue to the brown wire and that seemed to do the trick. Thanks everyone.
    – techguy817
    Commented Jun 2, 2013 at 0:25

I'm an electrical site supervisor in Dubai. This is the same problem that happened in my site also. It's actually quite simple to connect common C wire:

Wires from outdoor unit:

  1. Common wire - brown or blue color
  2. Cut-off switch wire (in my unit it's blue color)

Wires from indoor fan relay:

  1. Red (24VA power)
  2. Brown (common "c") in your case it's blue
  3. Gray (fan relay control)


  1. Connect outdoor unit cutoff switch wire (blue wire) to thermostat "y"
  2. Connect outdoor unit common wire and indoor unit common wire and connect to thermostat "c"or "x"
  3. Connect 24VA red wire to thermostat "RC" or "RH"
  4. Connect gray (fan relay control) wire to thermostat "G"

I hope this will work for you.


After exhausting every possibility for a failing common I found a splice of old wire and new wire where the air handler was swapped out but not the thermostat. After connecting the thermostat common (blue) to the brown common in the air handler it still wouldn't work. I had to trace the wire until I found the splice and was able to tie the common together at that point. After that I had no problem.

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