Your boiler is failing to light because it cannot detect the flame, likely due to a sensor needing cleaning or replacement.

Electrode damaged or not correctly positioned

What causes this fault?

This fault occurs when the boiler's internal computer is unable to verify that the gas and air are mixing in the correct proportions for a safe flame. It is usually triggered because the sensors aren't detecting a consistent signal during the start-up phase or the gas valve isn't responding correctly to the calibration commands. This prevents the boiler from firing up to ensure no unburnt gas or dangerous fumes are produced.