Replace the battery, since this can avoid having to replace the entire unit.
Reset the programming of the thermostat if your thermostat is digital. Thermostats that can be programmed occasionally default to manufacturer's settings if they lose power, or if the system experiences a coding glitch.
Make sure that the electrical system connected to the thermostat is functioning properly and that there are no fuses blown. If the system is somehow compromised, digital thermostats may not be able to hold their settings, and non-digital thermostats may not be able to turn on the heating and cooling system even if the thermostat itself has no issues.
Turn off the power to the thermostat and heating and cooling system. You will need to throw the circuit breaker to make sure the power is off.
Take off the cover plate of the thermostat. It may just pop off, or you may need to unscrew it, depending on the model.
Note how the wires connect to the electrical terminals, and disconnect them.
Secure the disconnected wires with your wire clip. This will prevent them from getting tangled or slipping out of reach.
Unscrew and remove the base of the thermostat.
Prepare the new thermostat by threading the electrical wires through the new thermostat's base.
Level the thermostat with the level and screw the base into position.
Connect the wires to their appropriate terminals on the new thermostat.
Pop or screw the cover of the thermostat into position.
Turn the power to the thermostat and heating and cooling system on at the circuit breaker.
- There are many heating and cooling issues that may seem like thermostat issues but which really aren't (e.g., no heat circulating in a baseboard heat system due to a closed valve). You may want to note the other issues that have the same symptoms as a faulty thermostat before changing yours.