I have central air and heat, with a Nest thermostat. My system has Y, G, R, C and W wires coming out of it: circuit board

which are wired into my Nest like so: Nest wiring

In between, there is a splice where the C wire is spliced with electric and another wire runs from that splice back to the C hookup:

C splice with eletric

And then the other wires are spliced with a new set of the same color:

other splices

The heat and cooling work fine. I just can't get the fan to run on its own. What could be at fault and how can I diagnose it?

  • Where do the two wires on the Y terminal go?
    – Steven
    Commented May 1, 2013 at 2:48
  • @Steven One will go to the thermostat, the other goes to the A/C.
    – Tester101
    Commented May 1, 2013 at 10:56
  • Did this work with the old thermostat? In your third image where the C wire is spliced with electric and some other wire, where do those cables go/come from? Are you sure the Nest is configured properly? The G wire is the fan signal wire, so make sure that wire has continuity end to end.
    – Tester101
    Commented May 1, 2013 at 11:10
  • Progress update? Did you have to replace the board?
    – Mazura
    Commented Apr 11, 2015 at 1:04
  • It's also worth noting that the Nest will refuse to signal for fan if it is in the 'off' mode, even though it is physically possible.
    – antiduh
    Commented Jul 25, 2016 at 16:28

2 Answers 2


I would first try touching the R and G together at the thermostat board (the wires coming out of the wall) and seeing if the fan turns on. If it does, then all the wiring is correct. If it doesn't, then I would try to do the same thing with two short wires that have been attached to the thermostat control board R and G terminals. If it still doesn't, then the problem is in the furnace. If it does, then you have a loss of continuity somewhere between your thermostat and your furnace on the G wire (because the heat and A/C work, so the R works). I don't have a Nest myself, but I know that not all thermostat systems make it obvious how to run the fan without cooling or heating.

  • 1
    R + G makes a clicking sound in the furnace, but produces no other sounds or airflow. Tried it directly on the board as well, with the same result.
    – noah
    Commented May 25, 2013 at 23:02
  • If you've got a click in the furnace, that more than likely means that the relay that controls the furnace is engaging. However, the fan itself isn't spinning, so it's likely between the relay and the fan (or the fan itself?). Do you have a wiring diagram for the furnace?
    – Aaron
    Commented May 29, 2013 at 18:36
  • The fan works when other functions of the circuit board call for it, just that the separate 'g' contacter isn't 'relaying' its signal, which means to us mortals: replace the board.
    – Mazura
    Commented Apr 11, 2015 at 1:02

It's also possible that if you hear a clicking sound, the relay is activating but the motor isn't connected to the switch. My blower motor had multiple lines for various speeds. When the Nest activates the heat, the fan would turn on after 30seconds, but that circuit is independent of the relay. So I had to choose another cable to run to the relay.

Then when I shorted the wires on the wall, it worked!

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