HVAC Controls 26408-23 Practice Test

When a cooling call triggers all three components—the compressor in the outdoor unit, the outdoor fan motor, and the indoor fan motor—none respond, the common thread is the low-voltage control power. The thermostat and the various relays in the furnace/air handler rely on a 24-volt supply from the control transformer to energize the outdoor contactor and the indoor blower relay. If that transformer fails, there is no 24 VAC available to close any of the relays, so the outdoor contactor won’t energize (no compressor or outdoor fan) and the indoor blower relay won’t energize (no indoor fan). In other words, without control power, the cooling call cannot activate any of the equipment. Other possibilities, like a thermostat failure, would typically affect only the signaling path or the thermostat itself, not necessarily cutting off power to all downstream relays at once. An outdoor sensor issue might prevent proper operation under some conditions but wouldn’t inherently stop all three motors on a cooling call. A blown fuse in the furnace could cut control power as well, but a transformer failure more cleanly explains loss of all low-voltage control circuits driving every component during cooling.