Happy new year! I was installing the Nest learning thermostat (3rd gen) and run into a "no power on Y1 wire" problem...it is saying no power on yellow wire, NEST error code is E80. It is winter I am located in NY, I am pretty sure the AC was working in summer if they are related... Any idea where could it be wrong on this one?

Here is my situation, I have 3 nest stat for 3 floors:

Floor 1: Nest thermostat E, no C wire installed, working fine so far..

Floor 2: Nest learning thermostat 3rd gen, no C wire, reporting E80 error - no power on yellow wire.

Floor 3: Nest thermostat E, didn't work without C wire, working fine so far after connected C wire.

I was thinking to put the "E" version device on the 2nd floor base to see if it reports same error but realized the E version's size is different with the learning 3rd gen version so can't do that...

Related question: Nest thermostat wiring to First Co air handler

  • Can you post photos of the wiring at both the thermostat and air-handler/furnace ends for the second floor 'stat please? Jan 1, 2021 at 22:52
  • You are fast and thank you! :) Sure will do it after dinner, I will probably try to connect the C wire like I just did for the E device while I get into the attic, meanwhile will take photos. I will update the post once I did it.
    – diabloooo0
    Jan 1, 2021 at 22:56
  • @ThreePhaseEel, Hi ThreePhaseEel, I just connected the C wire for 2nd floor nest and interestingly it works now! Though I am not really understanding why but that is all I need! Thank you so much for your help! Guess I will only have one last question, I don't use C wire on my 1st floor nest but it works so far (for whatever reason..), do you see a need to connect it to C wire? (that one actually might be easier because the air handler is located in basement so I don't need a ladder to the attic)
    – diabloooo0
    Jan 2, 2021 at 1:15
  • Yeah, using the C wire for the 1st floor Nest is better since that means you aren't relying on that Nest's internal battery any longer Jan 2, 2021 at 1:17
  • Also, post that last update as an answer and I'll give you a +1 for it since you figured out the problem on your own :) Jan 2, 2021 at 1:18

1 Answer 1


Problem solved - connect the C wire! But really the credit should go to @ThreePhaseEel because he helped me figure out which one was C wire from the air handler for the other nest thermostat installation.

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