A thermostat is to your HVAC system what a light switch is to your room lights, with the exception that the thermostat has an automatic switching mechanism — usually programmable — so you don't have to switch on the furnace or air conditioner manually. Since the thermostat is basically a switch, you can bypass it just like you can bypass a wall switch, and if the furnace or air conditioner is working properly, it should come on. This is the easiest way to determine whether the thermostat is responsible for the fact that your system isn't working.
Depending on the complexity of your heating/cooling system, the thermostat could have anywhere from two to six wires of different colors. The most important one is the one that carries 24-volt power from the system transformer, and it should be red. If you jump this wire to any of the terminals and the function associated with that terminal switches on, you have a bad thermostat. The most useful wire for diagnostic purposes is the one that sends power to the blower, which is usually green. The white wire, which switches on the heat, also operates the blower and can also be used.
How to Jump-Test a Thermostat
The power comes into the thermostat at 24 volts, which is a low enough voltage to handle without fear of a serious shock, but even so, it's a good idea to turn off the power while you're handling wires. Consequently, even though you could jump the power terminals in the thermostat with a spare length of wire, it's safer to disconnect the wires with the power off, twist them together and then turn the power back on.
Step 1: Switch Off the Power
Locate the switch for the furnace or air system and turn it off. If you can't find the switch, turn off the breaker in the main panel that controls the furnace.
Step 2: Expose the Thermostat Wires
Remove the thermostat cover, which usually just snaps off, and remove the temperature-sensing device by unscrewing the screws with Phillips screwdriver. Not every thermostat has a separate plate for the sensor; you may see the wires as soon as you remove the cover.
Step 3: Find the Power Wire
The power wire should be red and the blower wire green, but not every installer pays attention to the color code. A better way to identify the wires you want is to look at the identification letters on the terminals. The power terminal is labeled "R." If you have a heating and cooling system, the power terminals are "Rc" and "Rh." They are usually bonded together, and the wire is attached to the Rh terminal.
The blower wire is attached to the G terminal. If you want to test the heating system, use the wire attached to the W terminal. If you have an older two-wire thermostat, the wires will be red and white.
Step 4: Twist the Wires Together
Disconnect the wire from the R terminal and the one from the G or W terminal and twist them together with pliers. You don't need to cap the wires but be sure not to let any of them fall behind the wall.
Step 5: Turn On the Power
Turn on the switch or the breaker and wait a few minutes to see if the blower switches on. If it does, the thermostat is likely malfunctioning and should be replaced. If the blower comes on when you jump the G wire, you may want to repeat the test with the W wire to make sure the furnace burner comes on.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.