On this model a dirty flue venturi is a common cause. Google 'Vokera flue venturi', or look for postings on this site. Your boiler needs servicing by someone familiar with the model. He will clean the fan , heat exchanger and venturi, as well as perform other bits of preventative work (check pressures etc).
Contact vokera (www.vokera.co.uk) for a local approved man. Cheaper to get it fixed properly first time by someone very familiar with the model rather than by a corgi generalist fault finding by replacing parts at your expense.
c, November 2007
