A thermostat is simply a switch: inside it is a mechanism that activates whenever it detects a too-high or too-low temperature. And because it’s a switch, you can turn it off.

First, make sure the thermostat is set to ON, and set on either A/C or Heat. See if there are any programming overrides. Finally, replace your batteries if your unit is battery-operated.

Still not at “room temperature”? Now it’s time to check the HVAC unit itself by bypassing the thermostat.

Supplies needed: masking tape and a pen or pencil, flat head screwdriver, Phillips head screwdriver

How to Bypass the Thermostat

1. Turn the circuit breaker for your HVAC system off.

2. Remove the thermostat cover plate with a flathead screwdriver. (Each model is different: some will have a cover plate that pulls off easily, while others require you to twist, push a tab down, or remove a few screws.)

3. Use a Phillips head screwdriver to remove the wall mounting screws from the thermostat.

4. Using masking tape, label the wires coming out of the thermostat. They correspond to the letters R, W, Y, G, and C on the terminals of the thermostat.

5. Carefully unscrew the thermostat terminals, then disconnect the wires from your thermostat. You can use some masking tape to secure the wires and keep them from falling back into the hole in the wall.

6. Connect the R and W wires. The other wires should not be in contact with one another. Turn the circuit breaker of the HVAC unit on; the furnace should turn on. Turn the circuit breaker off, and untwist the R and W wires.

7. With the R and G wires, repeat step 6. This should cause the fan to activate. Turn the circuit breaker to the HVAC unit off and untwist the wires.

8. If steps 6 and 7 caused the furnace and fan to activate, then you have a faulty thermostat. Replace your unit as soon as possible.

9. However, if the tests didn’t cause your furnace or fan to turn on, the HVAC system might be at fault. Make sure your unit is getting power; if it’s getting power but isn’t running, this could point to a break in the wire connecting the thermostat to the furnace or to a problem with the actual furnace. At this point, you’ll want to call a professional for an assessment.

Whatever the results, you just saved a chunk of change by doing these tests yourself.

10-Minute DIY: Bypass Thermostat to Test Your HVAC Unit