High Warning
A9 on Viessmann Vitodens 100-W

Your boiler has detected an internal electronic error and has safely shut itself down to prevent further issues.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Control mode. Internal fault during communication with the OpenTherm room thermostat.

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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Related fault codes

A7View →F2View →

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