No Power to the Thermostat

The thermostat is only a switch and a screen. If the screen is dead, it is not getting power. Some models steal that power from batteries. Most modern ones also expect a 24-volt feed from the air handler, often on the wire labeled C. Either source can be the one that failed.

Checks before any wire comes off

• Replace the batteries if the thermostat takes them, even if you think it is hardwired. A lot of them will not light the screen on the C wire alone when the batteries are dead, and some are the reverse.
• Turn the furnace or air-handler switch on. Confirm the blower door is seated. An open door switch kills power to the control, and the thermostat goes dark with it.
• Check the breaker. Reset it once. If it trips again, leave it off and book service.
• Look at the outdoor disconnect only to see that it is seated. Do not open the condenser panel.

What not to do at the wall

Pulling the thermostat off and twisting red to white, or red to green, is an old trick that can show whether the equipment will run. It also shorts the transformer the moment a bare conductor touches the wrong terminal, and the little fuse on the furnace board opens. Then you have a blank thermostat and equipment that will not start. Leave the wires where they are if you do not do this work.

A fuse that is already blown, a transformer that failed, or a wire chewed in the attic will not respond to new batteries. That is heating repair or cooling repair, depending on what died with the screen. Tell us the screen is blank and which of the checks above you did.

Common questions