Thermostat displays A8 and cannot control the heating valve or relay; boiler does not fire even though there is a heat demand.

Safety first: Turn off power at the breaker before touching any thermostat or boiler wiring. Verify absence of voltage with a suitable tester if you are qualified.

• Check wiring diagram: Compare the actual wiring to the Viessmann installation manual. Ensure the thermostat output is connected to the correct low‑voltage control terminals on the boiler (e.g. KM‑BUS or low‑voltage demand input), not directly to mains.
• Inspect terminal connections: Tighten any loose screws on the thermostat backplate and boiler control terminals. Look for burnt or discolored insulation indicating overheating.
• Test boiler input: If safe and you are competent, temporarily bridge the boiler’s demand terminals to see if the boiler responds. If it does, the thermostat output is likely faulty.
• Reduce load: Ensure no high‑current devices (e.g. pumps) are connected directly to the thermostat relay; they must be controlled via appropriate boiler or zone modules.
• Replace thermostat if relay is defective: If wiring is correct and the boiler responds to a manual bridge but not to the thermostat, replace the thermostat.