If the fan fault light is on then it could be a air flow fault on the boiler as the air pressire sensor is picking up a fault on the flue etc.
Turn the boiler off for a few minites, back on again, left hand dial anti clockwise to the R position (reset) then see if it clears. If not it'll be a gas safe registered plumber you'll be looking for I'm afraid as it might be the pressure switch.
Devon boy, November 2010