Right now I don't have specifics on my central air unit (make, model, etc) but hopefully the info I provide will help.

I have a Second Generation (and Third Generation) Nest Thermostat hooked up to the Central Air Unit. I receive intermittent messages of "not enough power" being sent to the thermostat. (The reason I mentioned both Nests is that I've tried both units).

The wire contains one hot wire (Rh), one for the fan (G), and one for the cool/compressor (Y1).

I used a multimeter to test the voltage, and when I touch the positive probe (red wire) to the Rh wire and the negative probe to the G1 wire, I get a strong enough reading (between 27-28 volts). But when I do the same to the Y1 wire I get next to nothing.

After some research, I found people recommending changing the fuse in the unit itself... but when I lifted the panel, I didn't see any fuses that could be replaced. Moreover, the power drain doesn't seem to be consistent (i.e. today it is showing a poor voltage reading and tomorrow it will be fine).

I don't believe its the Nest because I've tried different ones and get the same results. Sorry for the long question, but I wanted to be specific. Any ideas on what is causing the power generation problem?

  • We'll have to know the make and model of the air handler and condensing unit. It would also help if you could provide a clear photo or diagram of the wiring in the air handler.
    – Tester101
    May 17, 2016 at 13:08
  • OK thanks. I'll try to take as many pictures as I can.
    – mjt117
    May 17, 2016 at 13:29
  • If it's an A/C only system, you should have the "hot" wire connected to Rc not Rh. Though I'm not sure that really matters with the NEST.
    – Tester101
    May 17, 2016 at 13:30
  • It is an A/C only system. NEST recommends Rh but through troubleshooting tried seating the hot wire in Rc and yielded the same results.
    – mjt117
    May 17, 2016 at 13:34
  • How difficult would it be to run a new cable between the thermostat and the air handler? Are there any extra wires in the existing cable?
    – Tester101
    May 17, 2016 at 13:38

1 Answer 1


This is the most common problem with the Nest. You need to hook up a "C" wire.

Here's the thing. Originally, thermostats were passive devices - nothing more than switches. And so, controls were wired in a "switch loop" similar to how switches in houses were wired before current electrical code. Power went in a functional loop: from the 24V transformer, to the thermostat, to the actuator, and back to the transformer.

The R wire is sorta like a "hot" wire, the other wires at the thermostat are sorta like "switched hot". And just like a switch loop doesn't have or need a neutral, neither does the thermostat (the neutral would be the C wire).

Old style dimmers and motion sensors powered themsleves by leaking a bit of power through the incandescent bulb. That fails with efficient CFLs or LEDs. Likewise, the Nest tries to power itself without a C wire by leaking a bit of power through the big clunky old relay that switches on the furnace, fan or A/C. That fails with efficient relays or electronic controls.

In this situation, you need to run a C wire, as the Nest manual discusses. Tester101 is proposing that you may have spare wires in your thermostat cable.

There are other ways to solve the problem, but they're starting to resemble electrical engineering.

  • I'm having trouble seeing where the control board even is... I think it's within the same panel as the main air vent. It's a Luxaire Model No. NAMB-FED14AA
    – mjt117
    May 22, 2016 at 12:05
  • There may not be a control board. There will, however, be a transformer. It might be nearer the fan. There will certainly be a wire from the fan relay to the transformer, so if necessary, follow the wires from the fan to the relay to the transformer. May 22, 2016 at 19:52
  • Apologies, that was a misleading comment from yesterday and it should be ignored. I went back up into the attic and found the control board. So to clarify a few things: Re: running a common wire--this may end up being the answer but the reason I don't think this is the Nest doesn't always show the e74 (not enough power) issue. It comes and goes, which leads me to think of a loose connection at the control board because I've stripped and re-seated the wires within the thermostat terminals numerous times. I would think if it need the C-wire it would just show the error at all times.
    – mjt117
    May 23, 2016 at 16:03
  • I suspected the Y1 as the loose wire culprit since that was the one giving me the bad voltage reading with the multimeter. When I opened the unit this is how the wires were configured (FYI i removed the wire nut before taking the picture): goo.gl/vL5l8l So I assumed I would see the white wires connected to the control board itself rather than jumped together like that. It was the TIGHTEST inter-coiling i've ever seen so i cut off the ends and interlocked them and nutted it. FOR NOW the thermostat is showing a solid reading & see if it lasts. Model info: goo.gl/cMPjwN
    – mjt117
    May 23, 2016 at 16:08
  • There's nothing wrong with pigtailing off the control board, that way you can service the wires without messing with the control board. I would expect the Nest to indicate low power intermittently, that would vary by the state-of-charge of its battery. I can't see the pics, it hits me with a login challenge, but if a wire is not binding well, that would do it. Good chance you nailed it! May 23, 2016 at 22:23

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