Classic flue venturi problem that this and similar models suffer, google it. You are meant to get a corgi man to attend to it, or (if it is your own boiler in your house) be a 'competent person', as the flue Venturi is part of the safety system and is inside the sealed chamber.
You can diagnose it without opening the sealed chamber by looking for the air switch dropping out when the fan is on full, you need a multimeter an earth leakage breaker and great care that you dont electrocute yourself as there are mains voltages. The wiring schematic is in the back of the downloadable manual. The venturi needs to be spotless to work, build ups of grime or corrosion on its surface stop it working. Best left to a corgi man who knows this model and its typical faults well.
c
May 2009
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