While rare, thermostats do occasionally die, and knowing the symptoms could save you big time. Since most thermostats require minimal knowledge to install, correctly diagnosing a failed thermostat means you might not need to call a technician to get your system fixed.
Can You Get the System To Come On At All?
The first step in determining if the thermostat is the problem is to see if the issue is isolated to one system or another. It is highly unlikely that your air conditioner, blower, and furnace all died at the same time, so you should be able to get at least part of the system up and running. Since the blower is the only part of the system that you actually notice from inside your home, you'll have to actually check the furnace or walk outside and see if the fan on the condenser is running.
If that part of the system seems to be completely dead, don't give up yet. Flip the thermostat to the other side, whether to heat or cool, and change the temperature so that this second device may come on. You only need to run the system for a few minutes to determine if it will turn on. If your thermostat is the problem, then you won't be able to get anything to turn on at all.
Check the Wiring
The next step in diagnosis is to go to the breaker panel and check the lines for your air conditioner and heater. Obviously, if they are off, that is the primary cause of your issue. If everything seems to be in order, turn them off now. This will allow you to safely inspect the wiring between your thermostat and your HVAC system.
The front panel of your thermostat should pop off easily. Once you've done this you can make sure that all the wires are securely fastened. Do the same at the heater and air conditioner themselves. If there is a loose connection, address it and then turn the breaker back on to see if your issues have been resolved.
If the wires were loose, they may fall off completely when you remove the panel. Most technicians will wire the red and white pair for heat, and the red and green pair for air.
Remove the Thermostat From the System
If getting the wires tightened up doesn't help your situation, there is one last thing you can do to test to see if the issue truly is your thermostat. With the breaker off, remove the wires for either the heater or the air conditioner from the back of the thermostat and twist them together. This will complete the circuit, which is how your thermostat is supposed to tell the system to turn on. Once you flip the breaker for that appliance, it should be able to start up. If this doesn't work, turn the system back off from the breaker (it would continue to run constantly if you didn't) and get a new thermostat. If that system still won't come on, you can try the same experiment with the other side. This will rule out a problem with both your HVAC appliance and your thermostat.
If you can't get either system to come on, even without the thermostat in the system, it is definitely time to call a tech, as there is something seriously wrong with your HVAC system. More than likely, however, the thermostat is to blame and you can solve your thermostat is to blame and you can solve this with a quick trip to the home improvement store for a new one.
While it is true that installing a thermostat is usually a simple operation, it still isn't for everyone. Don't be afraid to call in a tech even if the problem turns out to be this simple. The cost of professional installation and testing will be far less than the repair bill caused by improper installation.