BoilerFaultCodesWhat the manual says
Manufacturer manualControl mode. Internal fault during communication with the OpenTherm room thermostat.
From this model's manual
Verified against the Viessmann Vitodens 100 W installation & service manual.
What causes this fault?
This fault occurs when the boiler's main control board loses its digital connection with the room thermostat. The most common reason is a loose or damaged connection wire, though it can also be caused by a software glitch or a hardware failure within the thermostat itself.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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