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Heater or A/C won't turn on
Check for a blown fuse or tripped circuit breaker
Make sure thermostat is actually set to "heat" or "cool"
Make sure the fan is set to "Auto"
Replace the battery in the thermostat, even if the display lights up
If one will turn on but not the other, troubleshoot that equipment
Thermostat doesn't come on at the right temperature
does not turn off at the right temperature
Mechanical thermostats need to be level to operate correctly, make sure it is level
Double check that it is programmed correctly
Make sure that the thermostat is not affected by drafts from vents, windows or other sources
Thermostat won't retain programmed settings