I swapped out a 24v Honeywell Thermostat for a NEST Thermostat. I followed the Manufacturer instructions for install. Everything was working for a week or so. I recieved an error(pictured below) regarding the Y1. NEST indicated I must need a "C" Common Wire run from the air handler.

Can I Tap any of the wires, going to the outdoor unit?

Blue/ Brown- Going to TACO sr503 controller Yellow/Green/ White /Red- Going to NEST GREY SHEATHED (Not Pictured) - Going to Outdoor Unit

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  • It's difficult to say for sure what's going on, since there's no context in your second photo. It just looks like a bunch of wires connected together. Could you label each cable, so we know where they go? Also, it's difficult to tell from the photo how the wires from the two coiled up cables are connected. Could you reposition them a bit, and snap a new photo? Including a photo of how the thermostat is wired would also be helpful.
    – Tester101
    Commented May 21, 2015 at 17:10

2 Answers 2


According to that wiring diagram, C is not available on the terminal block for the thermostat and outdoor unit. You'll have to find it by locating the 24V transformer. According to the wiring diagram, the yellow wire on the transformer would be C. You can verify that it is the correct wire because it should be connected to a couple of relays, and the other wire coming off the transformer should be red and connected to the terminal block for the thermostat.

  • Maybe it goes without saying...but make sure the breaker that goes to your furnace is OFF before you go routing around inside there looking for the C wire. Commented Aug 20, 2015 at 13:20

This is not so much an "answer" as just some background info on what a C wire is for. (and why it sometimes isn't there where you need it) I wrote this a couple months ago and have been posting it whenever I see one of these (frequent) C wire questions.

The lowdown on C wire

This is my attempt to "demystify" the whole "C" wire thing for non-technical folks. This explanation intentionally ignores lots of details that are not relevant to a basic understanding of the concept.

An old-timey thermostat is mechanical in nature. You can think of it very much like 3 separate mechanical on/off switches. (the "switches" in this case are mechanically actuated by temperature using springs and tilting glass bulbs full of mercury and other old-timey mechanical mechanisms) The circuit is actually VERY simple. The old-timey thermostat switches on and off the "juice" which comes in on the RED wire...separately on and off to each of the other 3 wires (often green for the fan, white for the heat, and yellow for the AC).

That's all the old-timey thermostat does. Hook or not hook the red wire to each of the other three as needed.

Modern thermostats control your HVAC system with the same connections. They do the same thing...hook the red wire up to the proper green/white/yellow to turn various parts of you HVAC on and off as needed.

Since the old-timey thermostats were mechanical in nature, they didn't need any juice of their own to operate. If you think of a modern thermostat as an old-timey thermostat plus a smart phone stuck to the wall right next to it...you need something to power the smartphone. (some "kind of modern" thermostats have batteries in them for this purpose...to power the "smarts")

The red wire brings juice into the thermostat...but there is no "ground wire". Our "smartphone stuck on the wall next to the old-timey thermostat" needs both a "hot wire" and a "ground" (or "common"). (again, this is simplified so as to not get lost in irrelevant details and confuse the non-technical readers this was written for)

The "C Wire" (often blue) is this additional "ground wire" needed in order to complete a circuit for the electronics (which old-timey thermostats didn't have). The "other end" of the C wire is hooked to the other side of the transformer that supplies juice to the R or red wire. It is the other wire needed by the electronics in the newer thermostats in order to power up their "smarts".

Hope this helps a few people understand what the C wire is about.

  • 2
    This doesn't answer the question.
    – Tester101
    Commented Aug 20, 2015 at 13:32

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