Emergency Lockout
FC on Viessmann Vitodens 100-W

Your boiler has detected a failure in its internal gas control system, meaning it cannot safely start the burner to heat your home or water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description

Burner in fault position. Modulation coil faulty. Check the gas train.

What causes this fault?

This fault occurs when the boiler's control system loses the ability to precisely adjust the amount of gas entering the burner. The most common cause is a failure in the electrical coil that moves the internal gas valve, meaning the boiler can no longer maintain a safe and steady flame to heat your home.

Engineer required

This is a emergency severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
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Related fault codes

F4View →F8View →

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