potterton profile will not fire up pcb working, air pressure switch working?

when i switch on boiler not a lot happens, first i thought it was the pcb board but thats not the problem, tried different air pressure switch but no good.... one thing i did notice is if i take tube off air pressure switch and blow into it the fan comes on and boiler starts up and runs fine until it reaches temp but then it wont start up again unless i blow on pressure switch again, also their is no voltage to fan until i blow on air pressur switch, i would have thought the fan needed to come on first to activate the air pressure switch ????