Can anybody help with a Vaillant boiler with a fault code f33 ?

f33 faults on valliant first thing to check is the air pressure switch its on top of the boiler right hand side,you need a short bit of flexible tubing to fit on to it then you take one pipe off it,either blow or suck the pipe you put on it,if you hear it clicking its ok and then you know it is youre fan we have put 3 fans in three years and it looks to me as if they are puting in sub standard fans

Bryan gas service engineer
May 2010