I had this problem, it was caused by the air pressure switch not working. This can be because the fan is not working, but sometimes they just stick. If you bang the left hand side of the outer casing near the bottom while the system is calling for hot water it might clear the problem, temporarily at least. The switches are pretty expensive to have replaced, so it is worth a try. (No qualification needed for this approach!!)
Fuelsaverman, May 2008