Nest thermostat c wire

[Sc72]

I installed my Thermostat 9 months ago.   Today all of a sudden it’s telling me I need a Wire installed on the “C” connection.   I keep getting error codes.   For 9 months this was not a problem.  There are no other wires to connect.   So why is this a issue now.

Any thoughts, feedback is welcome.

[StanGale]

Just bought a nest thermostat and have been told that you don't need a C wire.

However, many posts contradict this.

Is Google going to give us an official position on this? Why do they sell a "Nest Power Connector" if a C-wire is isn't necessary?

Why hasn't Sc 72s question been answered?

Can't install this without reasonable expectation that it's going to work without troubles.

 

[Sc72]

So apparently Nest Thermostats need 5 wires to work correctly.   It doesn’t tell you that when you buy it.  The 5th wire is the wire that charges the battery back up….  Without charging the unit, the battery backup dies in about 9 months (depending on usage of course).  As you are running the Thermostat off the battery backup.  
Most houses built before 2020 (at least in my area) only have 4 wires.  
Good luck fishing a new line from the furnace to the hole in the wall where your Thermostat is located. As the builders tend to follow building code and staple the wire to the studs behind the drywall. 
My in-laws had the same issue on their 4th generation thermostat a few months back.  It lasted just over a year, before it died.

I am now the owner of 2 Nest Thermostats that I can’t use. 🤪.  One 3rd generation and 1 Fourth generation.

 Oh and if you want you can just pull the 3rd generation unit off the wall and slide a new AA battery in it. And replace the unit.  Make it part of your routine when you change your smoke detector batteries…