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Similarly to this question of the same name, I am reading low voltage over the R,C wires at the thermostat. Unlike that question, however, the voltage is unusably low. I am only reading 2V over R,C which is low enough that my Nest 3.0 thinks the power is completely disconnected. All of the other connections (R,Y, R,G, and R,W) read 28V. I have a 2nd thermostat downstairs (the one previously measured is upstairs, closest to the furnace in the attic), and it has the same problem/measurements.

R,C R,W

If I go over to the furnace (York Furnace Model P3HUB16L06401D) and measure the voltage across the wires, not only is everything else 28V, but R,C is also 28V.

R,C

The two new Nest thermostats I am installing are replacing two older traditional thermostats that didn't have any backlight/special features, so they didn't use the C wire. The C wire was already connected on the furnace side, but it was left loose on the thermostat end, so if there are issues with the original C wire installation, it could have easily gone unnoticed until now. It's hard to see in the first set of images, but on the thermostat side, all of the wires are coming out of the same brown casing seen in the furnace.

What could cause the C wire to have such a low voltage on the thermostat end of the wire, but allow all of the other wires in the casing, and even the end of the C wire at the furnace, to behave normally? Could the C wire alone somehow have gotten damaged somewhere in between the furnace and the thermostats & now the whole set of wires needs to be reran & replaced? I do not believe it is the issue mentioned in this question where the insulation is shoved into the connection, as the following images show the insulation has plenty of clearance. I also made sure the connection was tight.

R,C

Edit: The problem is indeed what is mentioned in @JPhi1618's answer below. The original technician only hooked up the C wire to the furnace, and not the outbound wires to the thermostats inside the zone controller. You can see in the image below that the C wire is just wound around the outgoing wires for zones 1 and 2 on the left side. enter image description here

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  • Is running a new thermostat cable an option? Sep 14 at 11:37
  • Is that green wire touching the red wire base?
    – gbronner
    Sep 14 at 14:38
  • @gbronner Ha, sure looks like it, but if it was, the question would be "Why is my AC fan always running no matter what?"
    – JPhi1618
    Sep 14 at 16:01
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The only thing that could be happening in this case is a broken wire, or a bad splice. A solid, continuous wire will have voltage on the other end, even if its partially damaged. A damaged wire will not have the current carrying capacity (amps) that a good wire does, but it will show voltage. Also, since this is alternating current, the 2 volts you're measuring is most likely an induced phantom voltage.

What has probably happened is there is a splice somewhere in the wire, and the person that spliced it didn't think the blue wire was important. Follow the wire as far as you can coming from the air handler and see if you can find it. Also, gently tug the wires coming out of the wall near the thermostat. A small amount of wire could have been added near the thermostat if it was relocated. An inspection camera might help you see in the wall if that's the case.

The other option is that the splice is hidden and inaccessible because of an accident when the house was built. There are devices that can "add" a C-Wire over the existing thermostat wires, so that would be an option for you as well.

Nest power adapter: https://www.amazon.com/Google-Nest-Power-Connector-Accessories/dp/B093TRFQWP/ (you'll have to check to make sure this works with your model, link provided as an example of what to look for)

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  • "the person that spliced it didn't think the blue wire was important" was indeed the problem. I added an image of the zone controller to the question where the technician only hooked up C to the furnace and not the outbound zones. So fixing it would be as easy as plugging in the C wire to those
    – Drew
    Sep 18 at 23:50

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