Your boiler is unable to start because the small starter flame is not lighting, likely due to a tiny blockage in the gas supply line.

Pilot fails to light - Blocked Pilot Injector.

What causes this fault?

This fault occurs when the tiny orifice in the pilot injector becomes restricted by microscopic debris or carbon buildup, preventing the starter flame from reaching the sensor. Without this stable pilot flame, the boiler’s safety system refuses to open the main gas valve to prevent unburned gas from escaping. The most common reason is simply age-related wear or dust being drawn into the pilot assembly over time.