Your boiler is struggling to light its flame on the first try, though it may eventually start after several attempts.

Unsuccessful ignitions in the first attempt

What causes this fault?

This fault occurs when the boiler's internal computer detects that the gas has failed to ignite quickly enough during its initial startup phase. The most common reasons are a worn ignition lead failing to create a spark or a build-up of carbon on the sensing probe which prevents the boiler from 'seeing' the flame it has just lit.