Your boiler is unable to control the gas flow properly because of a communication fault between its brain and the gas valve.

Gas valve modulator connection is broken – could be due to an internal failure.

What causes this fault?

This fault occurs when the boiler's main control board loses the ability to send electrical signals to the gas valve. The most common reason is that the wiring connecting these two components has become loose or damaged, or the internal coil within the gas valve itself has failed. Because the boiler can no longer safely regulate the amount of gas entering the burner, it shuts down for safety.