Vaillant Turbomax constant ignition?

I have a Turbomax 242 with an intermittant fault which has now become permanent. when the boiler is switched into operation either by hot water or central heating the ignition starts, but does not switch off even when the flames are up it continues to try and ignite. This lasts for a few seconds and then the boiler shuts down and goes into error. In the past I have disconnected the circuit boards and reseated them which seemed to cure it so I suspected a dry joint. I have resoldered the connections on the PCB where the plugs are and any other susspect areas but the fault remains. Is this a red herring or do I need two new boards?
Many thanks for any advice.