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Can anyone help with my c wire blues with my Google Nest? I have two ACs, one for downstairs and another one for upstairs. I recently upgraded my battery operated t-stats with Nest. My old ones used only 4 wires, the blue wires were tucked away. On the first one, after connecting all 5 wires, to include the blue unused wire at the furnace and the tstat, Nest recognized all wires and is working fine. The second one, not so. It shows no power detected at C terminal. I did all the troubleshooting that I found all over the internet to no avail. I swapped the green and blue wires at both the furnace and the tstat to see if the blue wire is damaged, it works. I swapped the tstats same issue. I returned both tstats and got new ones, same problem. Update: I will upload the right images when I get back home. But to answer your question, with the multimeter at the furnace from R to all individual connections (Y W G AND C) it reads 28v. At the tstat level, all read the same with the exception of C that reads very low voltage, like 1v or some other low number. The culprit model is a Goodman that was installed in 2017. The furnace model is GMS80804BNBE. Coil and compressor is a 16 seer 4 ton.

                               ****** SOLUTION******

I went and bought a new 18-5 wire and connected it to the furnace and tstat. It worked!! Now when I was in the process of removing the old wire to replace it with the new one, I discovered that the old wire was not one continuous wire, it was two wires joined with wire nuts... And, you probably guessed it, there were only 4 wire nuts. The blue wires were not joined!! I don't know why someone would do that, but hey, it all ended well for me. Thanks all for your suggestions and contributions.

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  • What make/model is the culprit furnace? Also, do you have a multimeter? Oct 4, 2021 at 11:42
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    It appears you've uploaded the same image twice. Please edit your post to include the 2nd image I presume you intended to include.
    – FreeMan
    Oct 4, 2021 at 11:51
  • When you post the correct second picture, can you also fix both pics so the two cables are disentangled? One of the two brown cables at the bottom has two wires in use (from compressor). Should be red to Y and white to C. Would be nice to see that clearly. Then, if you don't have a voltmeter, buy a $20 one and measure the R-C voltage at the A/C and at the Nest base for both systems.
    – jay613
    Oct 4, 2021 at 11:55
  • I will upload the right images when I get back home. But to answer your question, with the multimeter at the furnace from R to all individual connections (Y W G AND C) it reads 28v. At the tstat level, all read the same with the exception of C that reads very low voltage, like 1v or some other low number. The culprit model is a Goodman that was installed in 2017. The furnace model is GMS80804BNBE. Coil and compressor is a 16 seer 4 ton Oct 4, 2021 at 18:37
  • Can you double-check to make sure your C wire is making good contact to the terminal screw? Also, does the AC turn on when you jumper R to Y? Oct 5, 2021 at 3:02

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I went and bought a new 18-5 wire and connected it to the furnace and tstat. It worked!! Now when I was in the process of removing the old wire to replace it with the new one, I discovered that the old wire was not one continuous wire, it was two wires joined with wire nuts... And, you probably guessed it, there were only 4 wire nuts. The blue wires were not joined!! I don't know why someone would do that, but hey, it all ended well for me. Thanks all for your suggestions and contributions.

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