Your boiler has detected a general internal error and has locked itself out for safety, meaning your heating and hot water will not work until it is reset or repaired.

The appliance has detected a fault (refer to manual section 3.3)

What causes this fault?

This fault occurs when the boiler's internal computer detects an inconsistency in the ignition process or a failure in one of its safety sensors. The most common reasons are that the boiler is struggling to generate a spark to light the gas, or a faulty sensor is sending incorrect temperature readings to the main control board.