it is easy to test if the aqua sensor is faulty (which would be very unusual) switch the heating demand and the pre heat(if activated) off and run a hot tap , if boiler fires up immediately aqua sensor is OK, there were problems with the origional diverter valves but if this has been replaced the newer ones are pretty reliable, do you know if you live in a hard water area? could be a lime scale problem
Ian, April 2010
