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Diagnose Thermostat Problems

Problems with a furnace or A/C may not be problems with the appliances themselves. Instead the controls may be at the root of the problem. Review the diagnostic table for some clues to possible causes.

Click on underlined items for further information
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"
  • Replace the battery in the thermostat
  • If one will turn on but not the other, it may be an equipment or wiring problem, but the thermostat is not ruled out
Thermostat doesn't come on 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
  • Replace the battery
Thermostat won't retain programmed settings
  • Replace the battery
Heater or A/C won't turn off automatically
  • Sensor may be blocked, clear away dust and anything blocking air flow into the thermostat
  • Thermostat may be located in a draft or is too isolated from the rooms being heated

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