High Warning
6F:100 on Viessmann Vitodens 300-W

Your boiler's computer memory card is telling the system to run at full power for hot water, which it cannot adjust or override.

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

Max. output during DHW loading 100%, defaulted by the boiler coding card

What causes this fault?

This fault occurs because the boiler's internal memory chip, which stores its specific performance settings, has failed or lost communication with the main computer. Without these specific instructions, the boiler enters a 'safe mode' where it lacks the data to modulate its power, leading it to default to maximum output when heating your water. It is usually caused by a physical hardware failure in the coding card or a fault on the main control board.

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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