Your boiler is failing to light the gas flame properly, meaning it cannot heat your water or radiators.

No ionisation after ignition phase, defective igniter / fan is not rotating / igniter and fan are functioning.

What causes this fault?

This fault occurs when the boiler's control system tries to start the burner but cannot detect a stable flame. It is usually caused by a blockage in the burners, a worn-out ignition spark, or a failed sensor that cannot 'see' the fire once it has lit. In some cases, it may also happen because the internal fan isn't spinning fast enough to clear out old gases and allow fresh air in for combustion.