Thermostat shows E4 and heating will not start; furnace or heat source does not respond.

Safety first: Turn off power to the furnace/air handler at the breaker before touching any wiring.

• Inspect W1 at thermostat: Remove the Wyze display and confirm the W1 (often white) wire is fully inserted into the W1 terminal and clamped securely. Re-strip if the copper is corroded or too short.
• Check W at furnace board: At the furnace control board, verify the same colored wire is attached to the W terminal. Tighten the screw and ensure no bare copper is exposed beyond the terminal.
• Verify adapter connections: If using a Wyze C-Wire Adapter, ensure the W wire from the thermostat is in the correct adapter port and the adapter’s W lead is on the W terminal at the control board.
• Continuity check: With power off and W disconnected at both ends, use a multimeter to check continuity of the W conductor. Replace or rerun the thermostat cable if the conductor is open.
• Restore power and test heat: Reconnect all wires, restore power, and set the thermostat to Heat with a setpoint above room temperature. The E4 code should clear and the furnace should start its ignition sequence.