Your boiler is currently going through its safety check phase before it attempts to light, which is a normal part of the startup process.

pre-purge stage

What causes this fault?

This fault occurs when water cannot move through the boiler quickly enough to carry heat away, causing the internal temperature to reach a dangerous level. The most common reasons are a failed external pump that isn't pushing water to the radiators or a buildup of sludge and debris that has physically blocked the pipes or heat exchanger.