Thermostat shows A01 and boiler does not fire for heating or hot water demand

Safety first: You are working with gas-fired equipment. Do not open the boiler combustion chamber or touch gas components. Isolate power before touching any wiring. If you smell gas, stop and call a qualified Gas Safe engineer immediately.

• Check boiler status: Go to the boiler and confirm it is actually showing A01 on its own display. If the boiler is in lockout, press the boiler's reset button as per the boiler manual and wait 2–3 minutes.
• Verify thermostat demand: On the Vokera smart thermostat, set the room temperature 3–5°C higher than current room temperature and ensure it shows a heating demand icon (flame or radiator symbol).
• Check wiring at boiler: Isolate power to the boiler. Remove the boiler front electrical cover (not the combustion cover). Locate the room thermostat / OpenTherm / low-voltage terminals. Ensure the Vokera smart thermostat wiring harness is firmly connected and no conductors are loose or broken.
• Check communication type: If your boiler supports OpenTherm and the Vokera smart thermostat is configured for OpenTherm, ensure the wires are on the correct OT+ and OT− terminals. If using simple on/off control, ensure the link wire has been removed and the thermostat is wired across the correct room stat terminals.
• Power cycle: Restore power to the boiler and thermostat, then perform a full power cycle: turn off at the fused spur for 30 seconds, then back on. Re-test heating demand.
• If A01 persists: The underlying cause is usually at the boiler (ignition, gas, flame sensing). Contact a qualified heating engineer; the thermostat is only reporting the boiler lockout.