Your boiler is not receiving the correct electrical power, meaning it cannot start the ignition process to provide heating or hot water.

Is there 240V~ between 12 and 10 and between 11 and 10? Correct power supply problem.

What causes this fault?

This fault occurs when the boiler's internal controls detect that the main electrical power is either missing or interrupted before it can reach the ignition system. The most common reason is an external fuse that has blown or a loose connection in the wiring that prevents electricity from flowing correctly through the unit. Without this steady power supply, the boiler cannot safely trigger the spark needed to light the flame.