High Lockout
F.27 on Vaillant Thermocompact

Your boiler has detected a flame when there shouldn't be one, which has caused it to shut down for safety.

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

No demand to gas valve | Electronic board defective; Gas valve defective

What causes this fault?

This fault occurs when the boiler's sensors detect a flame signal even though the gas valve should be closed and no heating is requested. The most common reason is a leaking gas valve allowing fuel to seep through, or a faulty circuit board that is incorrectly interpreting electrical signals as a flame.

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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