Your boiler is trying to light itself to provide heat, but it is failing to create a flame, likely because it is not getting enough gas or the spark isn't catching.

Start-up failure: Ignit. unsuccessful - Failure during start-up or ignition unsuccessful. Gas pressure monitor or thermal cut-out has been triggered.

What causes this fault?

This fault occurs when your boiler attempts to fire up but cannot detect a stable flame after several attempts. The most common reasons are a lack of gas reaching the unit, a worn-out spark plug that cannot ignite the fuel, or a internal sensor that is too dirty to 'see' the flame correctly.