F27 or e27 is the water level pressure switch problem. BUT can also be a blocked-up air trap. Connected to the p/switch is a rubber thin hose. If you follow that hose down you come to the air trap which most likely with a bit of luck is blocked up or part blocked up - clean that out if blocked and that should cure your problem if not then change the pressure swt. Go to siemens uk spares and type in your full Enr model number so you get the right part.