High
F4 on Viessmann Vitodens 300-W

Your boiler is failing to light the flame properly, meaning it cannot provide heating or hot water for your home.

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

Ignition: Check the ignition module (control voltage 230 V across plugs X2.1 and X2.2) / Gas combination valve opens: Check the gas train (control voltage 230 V) and gas supply pressure / Ionisation current builds (higher than 5 µA): Check electrode adjustment and gas pipe for airlocks

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

F2View →

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