Transformer
The 24-volt control transformer.
How to Test an HVAC Transformer: 24-Volt Checks, VA Rating and Shorts
How to test the 24-volt control transformer in a furnace or air handler, how to size a replacement by VA, and how to find the short that killed the last one.
Thermostat Blank or No Power: Finding the Missing 24 Volts
How to troubleshoot a blank thermostat: batteries, the C wire, the furnace door switch, a blown board fuse, a tripped float switch, the transformer and power to the air handler.
Furnace Control Board Fuse Blown (3A): Finding the 24V Short
Why a furnace control's low-voltage fuse blows and how to find the short in thermostat, outdoor unit or accessory wiring before replacing it.