Your boiler is failing to detect a flame when it tries to work at a high power level, causing it to shut down for safety.

No flame signal with high output

What causes this fault?

This fault happens because the boiler’s control system cannot detect a steady flame while the burner is trying to run at full power. It is often caused by a restricted gas supply, a worn-out ignition probe that can no longer sense the flame, or a physical blockage in the burner or flue system. Because the boiler cannot confirm it is burning fuel safely at high speeds, it shuts down as a protective measure.