ferrolli optimax 25c?

ferroli optimax 25c i have no hot water and no ch code f3, now is it, the diverting valve ?how can i check it ,i noticed that it moves up and down some times, when i run the hot water it runs hot for 10 seconds then it goes cold again, then the boiler makes a banging noise. then a (d) comes up on the boiler which is not a code, after that it will reset it self after 5 mins i can run the hot water on test, runs great but no ch this will only allow you to run for 15 min can anyone help paul