Thermostat displays E30 and reports a low-voltage short or blown fuse; HVAC system may be completely unresponsive.

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breaker. Do not attempt to bypass fuses with foil or larger fuses.

• Inspect thermostat wiring: Remove the Wyze display and carefully pull the wall plate away from the wall if possible. Look for any bare copper from the thermostat wires touching each other or the metal box. Re-strip and cap or tape as needed.
• Check Wyze C-Wire Adapter: If installed, inspect the adapter for melted plastic, burn marks, or damaged insulation. Replace the adapter if any damage is found.
• Inspect furnace control board fuse: Open the blower compartment and locate the low-voltage fuse (often a 3A or 5A automotive-style blade fuse) on the control board. If the metal link is broken or blackened, replace it with the same rating.
• Test without thermostat (advanced): With power off, disconnect the thermostat wires from the control board. Restore power and see if the fuse holds. If it blows again, the short is in the equipment; call a technician. If it holds, the short is in the thermostat wiring or thermostat.
• Reconnect carefully: After replacing the fuse and correcting any shorts, reconnect the thermostat wiring, restore power, and reattach the Wyze display. The E30 code should clear if the short is resolved.