Your boiler has detected a problem with how it monitors gas supply pressure, meaning it cannot safely confirm there is enough fuel to stay lit.

Gas pressure detector (if present) is defective or the wires have not been properly connected.

What causes this fault?

This fault occurs when the boiler's internal monitoring system loses contact with the sensor that checks for incoming gas. The most common reason is a physical failure of the pressure switch itself or a loose connection in the wiring that links the switch to the main control board. Without this signal, the boiler shuts down as a safety precaution because it cannot guarantee a steady flow of fuel.