In most homes, a digital 4-wire thermostat controls the heating and air conditioning (HVAC) system by sending a signal to turn on the heating unit, the cooling unit or the indoor fan motor. If one or more of the signals fail, you could be left without heat in the winter or cooling in the summer. Troubleshooting your digital 4-wire thermostat is a good first step when diagnosing your HVAC system.
Remove your thermostat from the wall plate by pressing or pinching the appropriate tabs and pulling it away from the wall. Note that all thermostats attach to their respective wall plates differently. Do not pry or yank your thermostat off the wall or permanent damage could occur.
Locate the four screw terminals to which the four thermostat wires connect. Typically a red wire will connect to a terminal designated as "R." A white wire will connect to the "W" terminal. A green wire will connect to the "G" terminal and a blue wire will connect to the "Y" terminal.
Connect one end of your jumper wire to the "R" screw terminal and/or wire.
Connect the other end of your jumper wire to the "G" terminal. The fan motor on your furnace should start and begin blowing air through your house.
Disconnect the jumper from "G" and connect it to "W." Your furnace fan should stop, and the heating unit should be in the process of ignition.
Disconnect the jumper from "W" and connect it to "Y." Your furnace should stop the ignition process and your outdoor air conditioner condenser should start.
If all of the above steps provided the expected results, yet your HVAC system does not operate with the thermostat connected to the wall plate, then your thermostat should be replaced.
If one or more of the above steps did not provide the expected result, the problem is elsewhere in your HVAC system.