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How to Troubleshoot a Two Wire Thermostat

Elizabeth Knoll

In time, your heating system may begin acting erratically. If it suddenly stops working properly, the problem may either be in the furnace or the thermostat. To determine the problem, some simple troubleshooting is necessary.

Once the source of the problem is found, you will be well on your way to having a comfortable home once again.

  1. Turn the power off to the furnace. This can be done by locating the switch on the side of the furnace and switching it to the "off" position.

  2. Snap the thermostat off the wall plate to gain access to the wiring. Disconnect the thermostat wires and separate them so they do not touch.

  3. Open your furnace control panel and disconnect the two thermostat wires from the control board. Connect the "R" and "W" terminals together with a short piece of thermostat wire.

  4. Replace the furnace access door and turn the power back on to the furnace. If the furnace turns on, either the thermostat was wired wrong, or you have a faulty thermostat. If the furnace doesn't turn on, the problem lies within the furnace and you should consult a qualified heating technician.

  5. Turn the power back off if your furnace worked and open the access door back up. Remove the jumper wire between the "R" and "W" terminal. Connect the two wires that you disconnected in step 3 back to the "R" and "W" terminals, and make note of what color wire is connected to which terminal.

  6. Connect the two wires back to your thermostat's "R" and "W" terminals. Check that the colors match. Turn the power back on to the furnace. Turn the thermostat on. If the furnace now operates properly, it was previously wired wrong. If it doesn't work, the thermostat is the culprit and needs replacing.

  7. Tip

    Rarely, the thermostat wire could have a short in the wall. If after replacing the thermostat, the furnace still doesn't operate, replace the thermostat wire.


    When working on the wiring, the power to the furnace must be off. If the wires improperly contact something, the furnace could be shorted out and damaged.