Room thermostat or external demand signal error, boiler does not respond to heat calls

Safety: Turn off power before working on control wiring.

• Check thermostat: Ensure the room thermostat or Viessmann Vitotrol is powered and set above room temperature.
• Inspect wiring: At the boiler, check the thermostat or demand terminals (e.g., 3 and 4). Verify that the wires are secure and not broken.
• Bypass test (only if safe and allowed): Temporarily link the thermostat terminals at the boiler to simulate a call for heat. If the boiler starts, the issue is with the thermostat or its wiring.
• Restore normal wiring: Remove any test links and reconnect the thermostat correctly.
• Replace faulty control: If the thermostat is unresponsive, replace it with a compatible Viessmann-approved control.