High Lockout
7F:1 on Viessmann Vitodens 300-W

Your boiler’s internal control system has encountered a rare hardware communication error that is preventing it from running properly.

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

Detached house

What causes this fault?

This fault occurs when the boiler's internal computer brain suffers a communication breakdown between its various electronic modules. The most common reason is a hardware failure within the main control board or a corrupted memory chip, preventing the system from processing safety-critical data. In some cases, it can be triggered by a sudden electrical surge or moisture ingress that has damaged the delicate circuitry.

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

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