Your boiler is detecting a flame inside the combustion chamber when it should be switched off, which is a safety concern that prevents it from firing.

False flame

What causes this fault?

This fault occurs when the boiler's internal electronics detect a flame in the burner area even though the gas valve should be closed and the system isn't currently heating. It is usually caused by a faulty sensor sending a 'ghost' signal to the control board or a malfunction in the ignition lead that creates electrical interference. In some cases, it can indicate that the main control board is incorrectly interpreting signals and has shut the system down as a vital safety precaution.