With this type of boiler, in my case it was, three times the fan assembly had seize up or the speed of the fan was not sufficent to operate the pressure switch to turn on the gas. Twice replaced complete fan assembly at nearly £99 a throw , other time just replaced the bearings.
The other time the ignitor white wire had come away from the spark electrode, since it is only push in. Cleaned wire and secured.
Raymond, March 2012
