Determining whether the low-voltage control fuse on your HVAC indoor furnace or air handler control board has blown involves observing total power loss to your wall thermostat. If your digital thermostat screen is completely blank, unresponsive, and fails to control your heating or cooling system even after you replace the batteries, a blown 3-amp or 5-amp automotive-style blade fuse on the indoor control board is a very common culprit. To check the fuse, turn off the power at the main circuit breaker, remove the furnace service access panel, and inspect the control board for the small plastic fuse. Pull the fuse out and inspect the internal metal wire; if the wire is broken or charred, the fuse is blown. You can verify that the fault is resolved when replacing the blown fuse with an identical amperage rating restores power to your thermostat display.
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