Your boiler has failed to light because it either cannot get enough gas or cannot detect a flame once ignited.

Interruption Of Gas Supply or Flame Failure (Gas supply interrupted, ignition failed or flame not detected)

What causes this fault?

This fault occurs when the boiler attempts to ignite but cannot establish or maintain a steady flame. The most common reasons include a lack of gas reaching the burner, a faulty ignition spark, or a sensor that is too dirty to detect that the flame has successfully lit.