What are wiring codes for a thermostat?

Although the wiring codes may vary from one thermostat to another, the basic wiring codes for a thermostat are red, yellow and green colors that show which wires connect to which terminals. Other thermostat wiring code colors include white, black and dark blue.

The red wire connects the power supply to the system circuitry and is attached to terminals that begin with letter R. Whereas the yellow wire is attached to the compressor and connected to the Y terminal, the green wire is connected to the G terminal, and it controls the fan relays that trigger the blower fan. For a proper functioning of the thermostat, these wires should be correctly connected to their respective terminals.