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All wires are connected but furnace won't turn on.

Community Member

Hello,  I have a gas furnace with ac.  I ran a plug in 2 wire 24v plug for the C wire power. 

The learning nest shows all the wires are connected and there aren't any error messages.  But during the test mode it won't turn on the furnace can at all. 

I can't connect with my phone as well. 


 This was the old thermostat. 


 This is inside the furnace. 



Red wire should connect to nest Rh slot. Also don’t see the C connected at your furnace. Do you have a 5th wire that you can use? Good luck. 

Community Member

I have the Nest wired like this

Y1- cool (blue in my system), G-green (fan), W1-white (heat), RH-red (power)

RC & C are connected to get power from the 24v (two wire charger, plugged into the wall)


Nest image.jpg

24 volt needs to be from furnace not external transformer. You already have the red wire connected from furnace to nest slot Rh just need another wire to go from furnace C / common to C on nest. If you can’t pull a wire or don’t have extra wire You can  use the green wire to go to C on nest but you need to swap at furnace also put green G wire to C also need to put small wire jumper at furnace between G & Y on the furnace where the blue wire is connected.   if you use green wire you lose ability to turn on fan only by itself but fan would come on with AC because of the jumper in between Y & G. Always Turn off power to furnace before working on it. Hope that helps. Good luck. 

Community Specialist
Community Specialist

Hi folks,


@ChrisVan, thanks for reaching out to us, and I'm sorry to hear about the situation. Please answer the following: 


  • Does the thermostat screen indicate that it’s actively calling for heating?
  • When did heating last work?
  • Did you select the correct fuel type?
  • Do the LED lights blink on the HVAC control board?
  • When was the last time you changed the furnace's air filter?
  • Have you experienced extremely cold weather lately?


Also, please gather the power readings of your thermostat on its Settings > Technical Info > Power (Battery, VOC, VIN, and lIN). 


I appreciate your help, @HVAC