Hi Paul, F4 is a non drain fault. If it does this after a few minutes, take off the plinth,take off the silver front cover and see if there is water in the base.If there is you will need a new recirculation motor and they are expensive.If there is no water it is more than likely the pressure switch.Quite cheap and easy to replace.If you need an engineer go to Diplomats engineers,they are very good.tel: 08457 959722.
All the best.
FOXY
August 2008