Thermostat display shows A0 and heating demand is not being sent to the boiler; room temperature remains below setpoint.

Safety first: Turn off power to the boiler and the thermostat circuit at the breaker before touching any wiring. Do not work inside the boiler cabinet unless you are qualified.

• Check KM‑BUS wiring: Remove the thermostat from its wall plate and verify the two KM‑BUS conductors are firmly clamped in the correct terminals (e.g. KM‑BUS 1 and 2). At the boiler control (e.g. Vitodens main PCB), verify the same two conductors are connected to the KM‑BUS terminals with no loose strands or corrosion.
• Verify polarity and continuity: Ensure the same conductor goes to KM‑BUS 1 on both ends and the other to KM‑BUS 2. If available, use a multimeter (power off) to check continuity of each conductor from thermostat to boiler.
• Inspect cable damage: Trace the thermostat cable along its route and look for crushed, cut, or pinched sections, especially near cable clips and wall penetrations.
• Power cycle system: Restore power to the boiler and thermostat, then power cycle both by turning off the breaker for 30 seconds and turning it back on. Wait several minutes to see if the A0 code clears.
• If still present: The boiler KM‑BUS interface or the thermostat’s communication circuit may be defective. At this point, contact a Viessmann‑certified technician to test the boiler control board and replace the thermostat if necessary.