The single most common reason a residential air conditioning system or heat pump refuses to turn on is a tripped electrical circuit breaker or a blown low-voltage control fuse located on the indoor furnace or air handler control board. Power surges, lightning strikes, or minor electrical shorts in the thermostat wiring can cause the small 3-amp or 5-amp automotive-style blade fuse on the control board to blow instantly, cutting off all 24V power to the wall thermostat and halting operation. Another extremely frequent culprit is a tripped float switch in the condensate drain pan, which deactivates the system to prevent water overflow damage. You can troubleshoot by checking your main electrical panel and inspecting the furnace fuse. You can verify that the issue is resolved when resetting the breaker or replacing the blown fuse restores power.
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