No c wire error

[Damoose]

I work in an old building that uses a dated thermostat. The thermostat has a yellow, red, white and green wire. Coiled around the wiring harness is a blue wire, which I assume is the C, but it doesn’t appear to be providing power when I connect it to the Nest. I’m assuming it’s disconnected at the furnace, which I can’t access since it’s for the building.

Regardless, my Nest has been sitting in a drawer for a year and had no power. I charged it for about five minutes, it turned on, and I connected all wires listed above except the C. It seemed to be working, but after a couple minutes I got an error that the system didn’t appear to be connected to power. It then went to a diagram menu that highlighted “C” in red, noting that I didn’t have a C wire.

My understanding was that this thermostat could work without a C. Is the issue that I only charged it for a few minutes? Or will that message come up after a couple of hours even if it’s charged?

Also, regarding that technical menu that had a diagram option, I received an error 159. I couldn’t figure out how to get out of this diagnostic menu and go back to the main thermostat functions. 

any help would be appreciated.

[Paul48312]

My nest has been working on 4 wires without a C wire for years. Then this week it stopped working. App says offline and 3rd gen thermostat states not enough power for WiFi. The solution is to use a C wire (common) to allow the thermostat full power to run the unit and charge the battery. Since I don’t run the fan only option I used the G wire and moved to C on the unit and the control board of my furnace. The alternative is to pull new 5 wire or add the power unit add-on.  When you don’t use a C wire the unit should scavenge power from the heating or cooling circuit when active. I suspect a recent software update broke this feature. 

[Paul48312]

Blue wire is normally the C wire which you have. Connect it at the Nest mount plate and verify it is connected at the furnace control board.

[zoeuvre]

Hi folks, 

Thanks for your input, @Paul48312. 

@Damoose, yikes! In order to provide power to your Nest Thermostat, it typically requires a common wire (C wire) connection. The C wire provides a continuous source of power to the thermostat. Without a C wire, the Nest Thermostat may experience power issues or display error messages.

While Nest Thermostats can operate without a C wire in some cases, it depends on the specific setup and compatibility of your HVAC system. In your situation, it's possible that the blue wire coiled around the wiring harness is indeed the C wire, but it may be disconnected at the furnace end. If you suspect the blue wire is the C wire, you can try to locate the other end of the blue wire at the furnace or air handler to see if it's properly connected. However, since you mentioned that you don't have access to the furnace since it's for the entire building, it may not be feasible to check the connection. On the other hand, if you cannot connect a C wire directly, you may consider using a Nest Power Connector. The Nest Power Connector acts as a workaround by using the existing wires to provide power to the Nest Thermostat. It typically involves connecting the Nest Power Connector at the furnace end to supply additional power to your Nest Thermostat. 

Let us know if that helps.

Best,

Zoe