High Lockout
F7 on Worcester Bosch Greenstar CDi Compact

Your boiler thinks there is still a flame burning even though it has commanded the system to turn off, causing 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

Flame error - False flame detected before burner start or Internal error

What causes this fault?

This fault occurs when the boiler's control system senses an electrical signal from the burner area at a time when there should be no fire present. The most common reason is a contaminated or damaged sensing electrode sending a false signal, or a fault within the main circuit board incorrectly interpreting electrical noise as a flame.

This fault can occasionally be triggered by high humidity or heavy rain, as moisture ingress into the flue or internal components can cause the electrical tracking that leads to a false flame signal.

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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