vaillant plus 837e?

Call a Gas Safe Register Installer in your area to rectify the fault. F33 is Fan/Air Pressure Switch. Check the flue is clear and if the fan spins but the boiler doesn't fire... it is probably the Air Pressure Switch.
Not a hard job to change- parts readily available.... no more than 10/15mins if it's the case. Parts are not expensive.
Had to do one this evening as my last job!!!!!
Hope this helps.

Neil-675
January 2010