Determining whether a residential thermostat is electrically "blown" or fried involves observing total operational failure and inspecting internal components for damage. Signs of a blown thermostat include a completely dead display screen that remains blank despite fresh batteries and confirmed 24-volt power supply from the HVAC unit, as well as an inability to trigger heating or cooling systems. When you remove the thermostat faceplate from the wall mounting bracket, inspect the circuit board and low-voltage wire terminals for black scorch marks, melted plastic, a sharp chemical burning odor, or bulging electronic capacitors caused by a severe power surge or lightning strike. You can verify that the thermostat is permanently blown when replacing batteries and resetting power fails to revive the device.
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