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? – ThreePhaseEel Jan 1 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 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 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 – ThreePhaseEel Jan 2 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 :) – ThreePhaseEel Jan 2 at 1:18

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.