Hello. My name is XXXXX XXXXX X'll do my best to help. Troubleshooting often requires meters and test equipment. Let's get started and see what we can do.
Lets open the furnace and look at the control board. Tell me what the five wires each connect to on the furnace board.We will go from here.
The C (Common) wire is optional on many thermostats but mandatory with the NEST.
Some thermostats using AA batteries do not need a common as the batteries are replaceable and provide power source,. However the NEST requires a 24 volt AC power supply. That is obtained through the R wire and the C wire.