Smart thermostat displays E03 and heating system does not respond to commands.

Safety warning: This involves mains-voltage boiler control wiring. Switch off power to the boiler and thermostat at the breaker before touching any wires. If you are not confident with electrical work, contact a qualified heating technician.

• Power down: Turn off the boiler and associated heating circuit at the main switch or breaker. Confirm the thermostat display is off if it is wired-powered.
• Inspect wiring: Remove the thermostat from its wall plate or extension kit cover. Verify that the COM and NO (or appropriate digital bus terminals, e.g., eBUS, OpenTherm) are wired according to the Tado installation guide for your boiler model.
• Tighten terminals: Check that no copper strands are loose and that each conductor is firmly clamped in its terminal. Retighten screws gently without over-torquing.
• Check for damage: Look for signs of burning, discoloration, or melted insulation on the thermostat backplate and boiler control block. If present, stop and call a professional.
• Confirm boiler input mode: Ensure the boiler is configured to accept the type of control used (on/off relay vs. digital bus). Misconfiguration can trigger E03.
• Restore power and test: Refit the thermostat, restore power, and request heat via the app. Listen for the relay click in the thermostat and check if the boiler fires. If E03 persists, the relay or control electronics may be defective.