my boiler is faulting with a flashing 50?

we have had a repair man out now a couple of times and he cant seem to find what the problem is as needs it to fault when he is there. boiler will work for a while and then will default and shuts down with the 50 indicator light flashing. this has been going on now for a couple of months and am worroying it is going to shut down completly. Any suggestions?