Your boiler is unable to control the gas flow properly, which is preventing it from lighting to provide heating or hot water.

Stepper motor fault (gas valve)

What causes this fault?

This fault occurs when the boiler's internal computer is unable to communicate with the mechanical motor that regulates gas flow. The most common reason is that the motor inside the gas valve has seized or failed, but it can also be caused by a loose electrical connection or a fault on the main control board.