we have had the same problem over the summer with our vokera combi boiler. We had the vokera engineer out and she changed the pcb (£125) and the heat exchange(£70) among other things. Best advice for you is to phone Vokera up and pay for their engineer to come out, the yearly plan may seem a bit steep in price but they will not leave until the problem is fixed properly, the price includes call outs fort he year, parts needed to fix boiler and labour of their own engineer. In our case they came out on three different days to make sure the boiler was working properly and for the next year if anything else goes wrong they come out free of charge within 24 hours of your call.
yvonne, September 2008