Has anybody come across same problem with a halstead ace high?

Halstead ace high boiler has just had its fan replaced, but when new fan installed boiler will not ignite unless combustion chamber door is open! when its closed and screwed up it will not go into ignition process! although the fan and pump are both running! thought it might be the air pressure swithch but with new one installed same problem! flue is clear both internal and external!! halstead engineers juat keep asking me to change parts!! can some one please help!!!