Your boiler has detected an electrical communication problem with the gas valve, which is preventing it from lighting safely.

Gas–valve control defective | Short circuit/earth (ground) leak in cable harness to gas valves, gas valve assembly defective (earth/ground leak from coils), electronic control system defective

What causes this fault?

This fault occurs when the boiler's main computer loses the ability to safely control the flow of gas into the burner. The most common reason is a breakdown in the electrical signals between the control board and the gas valve, often caused by a failing internal component or damaged wiring. Because the boiler cannot guarantee the gas valve will open or close correctly, it shuts down immediately as a safety precaution.